diff --git a/decode/qlook.py b/decode/qlook.py
index 53649f2..5406660 100644
--- a/decode/qlook.py
+++ b/decode/qlook.py
@@ -29,6 +29,8 @@
 DEFAULT_FORMAT = "png"
 DEFAULT_FREQUENCY_UNITS = "GHz"
 DEFAULT_INCL_MKID_IDS = None
+DEFAULT_OUTDIR = Path()
+DEFAULT_OVERWRITE = False
 DEFAULT_SKYCOORD_GRID = "6 arcsec"
 DEFAULT_SKYCOORD_UNITS = "arcsec"
 SIGMA_OVER_MAD = 1.4826
@@ -43,7 +45,8 @@ def pswsc(
     data_type: Literal["df/f", "brightness", None] = DEFAULT_DATA_TYPE,
     frequency_units: str = DEFAULT_FREQUENCY_UNITS,
     format: str = DEFAULT_FORMAT,
-    outdir: Path = Path(),
+    outdir: Path = DEFAULT_OUTDIR,
+    overwrite: bool = DEFAULT_OVERWRITE,
 ) -> Path:
     """Quick-look at a PSW observation with sky chopper.
 
@@ -58,6 +61,7 @@ def pswsc(
         frequency_units: Units of the frequency axis.
         format: Output data format of the quick-look result.
         outdir: Output directory for the quick-look result.
+        overwrite: Whether to overwrite the output if it exists.
 
     Returns:
         Absolute path of the saved file.
@@ -77,10 +81,11 @@ def pswsc(
     spec = da_sub.mean("scan")
 
     # save result
-    filename = Path(dems).with_suffix(f".pswsc.{format}").name
+    file_name = Path(dems).with_suffix(f".pswsc.{format}").name
+    file_path = Path(outdir) / file_name
 
     if format in DATA_FORMATS:
-        return save_qlook(spec, Path(outdir) / filename)
+        return save_qlook(spec, file_path, overwrite=overwrite)
 
     fig, axes = plt.subplots(1, 2, figsize=DEFAULT_FIGSIZE)
 
@@ -96,7 +101,7 @@ def pswsc(
         ax.grid(True)
 
     fig.tight_layout()
-    return save_qlook(fig, Path(outdir) / filename)
+    return save_qlook(fig, file_path, overwrite=overwrite)
 
 
 def raster(
@@ -111,7 +116,8 @@ def raster(
     skycoord_grid: str = DEFAULT_SKYCOORD_GRID,
     skycoord_units: str = DEFAULT_SKYCOORD_UNITS,
     format: str = DEFAULT_FORMAT,
-    outdir: Path = Path(),
+    outdir: Path = DEFAULT_OUTDIR,
+    overwrite: bool = DEFAULT_OVERWRITE,
 ) -> Path:
     """Quick-look at a raster scan observation.
 
@@ -134,6 +140,7 @@ def raster(
         skycoord_units: Units of the sky coordinate axes.
         format: Output image format of quick-look result.
         outdir: Output directory for the quick-look result.
+        overwrite: Whether to overwrite the output if it exists.
 
     Returns:
         Absolute path of the saved file.
@@ -174,10 +181,11 @@ def raster(
     cont = cube.weighted(weight.fillna(0)).mean("chan")
 
     # save result
-    filename = Path(dems).with_suffix(f".raster.{format}").name
+    file_name = Path(dems).with_suffix(f".raster.{format}").name
+    file_path = Path(outdir) / file_name
 
     if format in DATA_FORMATS:
-        return save_qlook(cont, Path(outdir) / filename)
+        return save_qlook(cont, file_path, overwrite=overwrite)
 
     fig, axes = plt.subplots(1, 2, figsize=(12, 5.5))
 
@@ -202,7 +210,7 @@ def raster(
         ax.grid(True)
 
     fig.tight_layout()
-    return save_qlook(fig, Path(outdir) / filename)
+    return save_qlook(fig, file_path, overwrite=overwrite)
 
 
 def skydip(
@@ -215,7 +223,8 @@ def skydip(
     chan_weight: Literal["uniform", "std", "std/tx"] = "std/tx",
     pwv: Literal["0.5", "1.0", "2.0", "3.0", "4.0", "5.0"] = "5.0",
     format: str = DEFAULT_FORMAT,
-    outdir: Path = Path(),
+    outdir: Path = DEFAULT_OUTDIR,
+    overwrite: bool = DEFAULT_OVERWRITE,
 ) -> Path:
     """Quick-look at a skydip observation.
 
@@ -236,6 +245,7 @@ def skydip(
             the atmospheric transmission when chan_weight is std/tx.
         format: Output image format of quick-look result.
         outdir: Output directory for the quick-look result.
+        overwrite: Whether to overwrite the output if it exists.
 
     Returns:
         Absolute path of the saved file.
@@ -255,10 +265,11 @@ def skydip(
     series = da_on.weighted(weight.fillna(0)).mean("chan")
 
     # save result
-    filename = Path(dems).with_suffix(f".skydip.{format}").name
+    file_name = Path(dems).with_suffix(f".raster.{format}").name
+    file_path = Path(outdir) / file_name
 
     if format in DATA_FORMATS:
-        return save_qlook(series, Path(outdir) / filename)
+        return save_qlook(series, file_path, overwrite=overwrite)
 
     fig, axes = plt.subplots(1, 2, figsize=DEFAULT_FIGSIZE)
 
@@ -273,7 +284,7 @@ def skydip(
         ax.grid(True)
 
     fig.tight_layout()
-    return save_qlook(fig, Path(outdir) / filename)
+    return save_qlook(series, file_path, overwrite=overwrite)
 
 
 def still(
@@ -286,7 +297,8 @@ def still(
     chan_weight: Literal["uniform", "std", "std/tx"] = "std/tx",
     pwv: Literal["0.5", "1.0", "2.0", "3.0", "4.0", "5.0"] = "5.0",
     format: str = DEFAULT_FORMAT,
-    outdir: Path = Path(),
+    outdir: Path = DEFAULT_OUTDIR,
+    overwrite: bool = DEFAULT_OVERWRITE,
 ) -> Path:
     """Quick-look at a still observation.
 
@@ -307,6 +319,7 @@ def still(
             the atmospheric transmission when chan_weight is std/tx.
         format: Output data format of the quick-look result.
         outdir: Output directory for the quick-look result.
+        overwrite: Whether to overwrite the output if it exists.
 
     Returns:
         Absolute path of the saved file.
@@ -325,10 +338,11 @@ def still(
     series = da.weighted(weight.fillna(0)).mean("chan")
 
     # save result
-    filename = Path(dems).with_suffix(f".still.{format}").name
+    file_name = Path(dems).with_suffix(f".still.{format}").name
+    file_path = Path(outdir) / file_name
 
     if format in DATA_FORMATS:
-        return save_qlook(series, Path(outdir) / filename)
+        return save_qlook(series, file_path, overwrite=overwrite)
 
     fig, axes = plt.subplots(1, 2, figsize=DEFAULT_FIGSIZE)
 
@@ -343,7 +357,7 @@ def still(
         ax.grid(True)
 
     fig.tight_layout()
-    return save_qlook(fig, Path(outdir) / filename)
+    return save_qlook(series, file_path, overwrite=overwrite)
 
 
 def zscan(
@@ -356,7 +370,8 @@ def zscan(
     chan_weight: Literal["uniform", "std", "std/tx"] = "std/tx",
     pwv: Literal["0.5", "1.0", "2.0", "3.0", "4.0", "5.0"] = "5.0",
     format: str = DEFAULT_FORMAT,
-    outdir: Path = Path(),
+    outdir: Path = DEFAULT_OUTDIR,
+    overwrite: bool = DEFAULT_OVERWRITE,
 ) -> Path:
     """Quick-look at an observation of subref axial focus scan.
 
@@ -377,6 +392,7 @@ def zscan(
             the atmospheric transmission when chan_weight is std/tx.
         format: Output image format of quick-look result.
         outdir: Output directory for the quick-look result.
+        overwrite: Whether to overwrite the output if it exists.
 
     Returns:
         Absolute path of the saved file.
@@ -396,10 +412,11 @@ def zscan(
     series = da_on.weighted(weight.fillna(0)).mean("chan")
 
     # save result
-    filename = Path(dems).with_suffix(f".zscan.{format}").name
+    file_name = Path(dems).with_suffix(f".zscan.{format}").name
+    file_path = Path(outdir) / file_name
 
     if format in DATA_FORMATS:
-        return save_qlook(series, Path(outdir) / filename)
+        return save_qlook(series, file_path, overwrite=overwrite)
 
     fig, axes = plt.subplots(1, 2, figsize=DEFAULT_FIGSIZE)
 
@@ -414,7 +431,7 @@ def zscan(
         ax.grid(True)
 
     fig.tight_layout()
-    return save_qlook(fig, Path(outdir) / filename)
+    return save_qlook(series, file_path, overwrite=overwrite)
 
 
 def mean_in_time(dems: xr.DataArray) -> xr.DataArray:
@@ -565,28 +582,52 @@ def load_dems(
     raise ValueError("Data type could not be inferred.")
 
 
-def save_qlook(qlook: Union[Figure, xr.DataArray], filename: Path) -> Path:
+def save_qlook(
+    qlook: Union[Figure, xr.DataArray],
+    file: Path,
+    /,
+    *,
+    overwrite: bool = False,
+) -> Path:
     """Save a quick look result to a file with given format.
 
     Args:
         qlook: Matplotlib figure or DataArray to be saved.
-        filename: Path of the saved file.
+        file: Path of the saved file.
+        overwrite: Whether to overwrite the file if it exists.
 
     Returns:
         Absolute path of the saved file.
 
     """
+    path = Path(file).expanduser().resolve()
+
+    if path.exists() and not overwrite:
+        raise FileExistsError(f"{path} already exists.")
+
     if isinstance(qlook, Figure):
-        qlook.savefig(filename)
-    elif (ext := "".join(filename.suffixes)) == ".csv":
+        qlook.savefig(path)
+        return path
+
+    if path.name.endswith(".csv"):
         name = qlook.attrs["data_type"]
-        qlook.to_dataset(name=name).to_pandas().to_csv(filename)
-    elif ext == ".nc":
-        qlook.to_netcdf(filename)
-    elif ext == ".zarr" or format == ".zarr.zip":
-        qlook.to_zarr(filename, mode="w")
+        ds = qlook.to_dataset(name=name)
+        ds.to_pandas().to_csv(path)
+        return path
+
+    if path.name.endswith(".nc"):
+        qlook.to_netcdf(path)
+        return path
+
+    if path.name.endswith(".zarr"):
+        qlook.to_zarr(path, mode="w")
+        return path
+
+    if path.name.endswith(".zarr.zip"):
+        qlook.to_zarr(path, mode="w")
+        return path
 
-    return Path(filename).expanduser().resolve()
+    raise ValueError("Extension of filename is not valid.")
 
 
 def main() -> None: