diff --git a/docs/design.md b/docs/design.md
index 0995ff18..9590af1f 100644
--- a/docs/design.md
+++ b/docs/design.md
@@ -12,11 +12,54 @@ We want to back up volumes, either by user operation or by periodic automatic pr
 
 ## Target
 
-- TBD
+- Ceph Block Device (RBD)
 
 ## Architecture
 
-TBD
+```mermaid
+%%{init:{'theme': 'default'}}%%
+
+flowchart LR
+
+    style Architecture fill:#FFFFFF
+    subgraph Architecture
+
+      classDef storageComponent fill:#FFFF00
+      classDef component fill:#ADD8E6
+
+      style PVCA fill:#90EE90
+      style storage fill:#FFFFE0
+      style Node fill-opacity:0
+
+      CD[CSI Driver]:::storageComponent
+      PVC[PersistentVolumeClaim]:::storageComponent
+      SC[StorageClass]:::storageComponent
+      PVCA[pvc-autoresizer]
+
+      kubelet:::component
+      Prometheus:::component
+      kube-apiserver:::component
+
+      subgraph Node
+        kubelet
+        storage
+      end
+
+      subgraph kube-apiserver
+        SC --- PVC
+      end
+
+      CD -- watch PVC --> PVC
+      CD -- expand volume --> storage[(Storage)]
+      Prometheus -- get metrics --> kubelet
+
+      PVCA -- 1. get target PVCs --> PVC
+      PVCA -- 2. get SCs --> SC
+      PVCA -- 3. get metrics of storage capacity --> Prometheus
+      PVCA -- 4. expand storage capacity --> PVC
+
+    end
+```
 
 ### How rbd-backup-system works