diff --git a/src/common/entities/EArbitraryPlannerItem.ts b/src/common/entities/EArbitraryPlannerItem.ts
new file mode 100644
index 00000000..b5dfbb69
--- /dev/null
+++ b/src/common/entities/EArbitraryPlannerItem.ts
@@ -0,0 +1,19 @@
+import { Prisma } from '@prisma/client';
+
+export namespace EArbitraryPlannerItem {
+  export const Basic =
+    Prisma.validator<Prisma.planner_arbitraryplanneritemArgs>()({});
+  export type Basic = Prisma.planner_arbitraryplanneritemGetPayload<
+    typeof Basic
+  >;
+
+  export const Details =
+    Prisma.validator<Prisma.planner_arbitraryplanneritemArgs>()({
+      include: {
+        subject_department: true,
+      },
+    });
+  export type Details = Prisma.planner_arbitraryplanneritemGetPayload<
+    typeof Details
+  >;
+}
diff --git a/src/common/interfaces/IPlanner.ts b/src/common/interfaces/IPlanner.ts
new file mode 100644
index 00000000..ce982d33
--- /dev/null
+++ b/src/common/interfaces/IPlanner.ts
@@ -0,0 +1,48 @@
+import { Type } from 'class-transformer';
+import { IsIn, IsInt, IsString } from 'class-validator';
+import { DepartmentResponseDto } from './dto/department/department.response.dto';
+
+export namespace IPlanner {
+  export interface ArbitraryPlannerItemResponseDto {
+    id: number;
+    item_type: 'ARBITRARY';
+    is_excluded: boolean;
+    year: number;
+    semester: number;
+    department: DepartmentResponseDto | null;
+    type: string;
+    type_en: string;
+    credit: number;
+    credit_au: number;
+  }
+
+  export class AddArbitraryItemDto {
+    // year, semester, department, type, type_en, credit, credit_au
+
+    @IsInt()
+    @Type(() => Number)
+    year!: number;
+
+    @IsIn([1, 2, 3, 4])
+    @Type(() => Number)
+    semester!: 1 | 2 | 3 | 4;
+
+    @IsInt()
+    @Type(() => Number)
+    department!: number;
+
+    @IsString()
+    type!: string;
+
+    @IsString()
+    type_en!: string;
+
+    @IsInt()
+    @Type(() => Number)
+    credit!: number;
+
+    @IsInt()
+    @Type(() => Number)
+    credit_au!: number;
+  }
+}
diff --git a/src/modules/planners/planners.controller.ts b/src/modules/planners/planners.controller.ts
index 2d5a9951..b2b939fc 100644
--- a/src/modules/planners/planners.controller.ts
+++ b/src/modules/planners/planners.controller.ts
@@ -9,6 +9,7 @@ import {
 } from '@nestjs/common';
 import { session_userprofile } from '@prisma/client';
 import { GetUser } from 'src/common/decorators/get-user.decorator';
+import { IPlanner } from 'src/common/interfaces/IPlanner';
 import {
   PlannerBodyDto,
   PlannerQueryDto,
@@ -44,4 +45,22 @@ export class PlannersController {
     const newPlanner = await this.plannersService.postPlanner(planner, user);
     return newPlanner;
   }
+
+  @Post(':plannerId/add-arbitrary-item')
+  async addArbitraryItem(
+    @Param('id') id: number,
+
+    @Param('plannerId') plannerId: number,
+    @Body() item: IPlanner.AddArbitraryItemDto,
+    @GetUser() user: session_userprofile,
+  ) {
+    if (id !== user.id) throw new UnauthorizedException();
+
+    const newPlanner = await this.plannersService.addArbitraryItem(
+      plannerId,
+      item,
+      user,
+    );
+    return newPlanner;
+  }
 }
diff --git a/src/modules/planners/planners.service.ts b/src/modules/planners/planners.service.ts
index bfb610b6..866d2fdf 100644
--- a/src/modules/planners/planners.service.ts
+++ b/src/modules/planners/planners.service.ts
@@ -1,11 +1,18 @@
-import { Injectable } from '@nestjs/common';
+import {
+  Injectable,
+  NotFoundException,
+  UnauthorizedException,
+} from '@nestjs/common';
 import { session_userprofile } from '@prisma/client';
+import { IPlanner } from 'src/common/interfaces/IPlanner';
 import {
   PlannerBodyDto,
   PlannerQueryDto,
 } from 'src/common/interfaces/dto/planner/planner.request.dto';
 import { PlannerResponseDto } from 'src/common/interfaces/dto/planner/planner.response.dto';
+import { toJsonArbitraryItem } from 'src/common/interfaces/serializer/planner.item.serializer';
 import { toJsonPlanner } from 'src/common/interfaces/serializer/planner.serializer';
+import { DepartmentRepository } from 'src/prisma/repositories/department.repository';
 import { LectureRepository } from 'src/prisma/repositories/lecture.repository';
 import { PlannerRepository } from 'src/prisma/repositories/planner.repository';
 
@@ -14,6 +21,7 @@ export class PlannersService {
   constructor(
     private readonly PlannerRepository: PlannerRepository,
     private readonly LectureRepository: LectureRepository,
+    private readonly DepartmentRepository: DepartmentRepository,
   ) {}
 
   public async getPlannerByUser(
@@ -93,4 +101,31 @@ export class PlannersService {
 
     return toJsonPlanner(planner);
   }
+
+  async addArbitraryItem(
+    plannerId: number,
+    body: IPlanner.AddArbitraryItemDto,
+    user: session_userprofile,
+  ) {
+    const planner = await this.PlannerRepository.getBasicPlannerById(plannerId);
+    if (!planner) throw new NotFoundException();
+    if (planner.user_id !== user.id) throw new UnauthorizedException();
+
+    const department = await this.DepartmentRepository.getBasicDepartmentById(
+      body.department,
+    );
+
+    const arbitraryItem =
+      await this.PlannerRepository.createArbitraryPlannerItem(planner, {
+        year: body.year,
+        semester: body.semester,
+        department_id: department?.id || null,
+        type: body.type,
+        type_en: body.type_en,
+        credit: body.credit,
+        credit_au: body.credit_au,
+        is_excluded: false,
+      });
+    return toJsonArbitraryItem(arbitraryItem);
+  }
 }
diff --git a/src/prisma/repositories/department.repository.ts b/src/prisma/repositories/department.repository.ts
index 30114ad0..76e4701e 100644
--- a/src/prisma/repositories/department.repository.ts
+++ b/src/prisma/repositories/department.repository.ts
@@ -6,6 +6,12 @@ import { PrismaService } from '../prisma.service';
 export class DepartmentRepository {
   constructor(private readonly prisma: PrismaService) {}
 
+  async getBasicDepartmentById(id: number): Promise<subject_department | null> {
+    return this.prisma.subject_department.findUnique({
+      where: { id },
+    });
+  }
+
   async getDepartmentOfUser(
     user: session_userprofile,
   ): Promise<subject_department | null> {
diff --git a/src/prisma/repositories/planner.repository.ts b/src/prisma/repositories/planner.repository.ts
index 041c23c0..9a6d8577 100644
--- a/src/prisma/repositories/planner.repository.ts
+++ b/src/prisma/repositories/planner.repository.ts
@@ -1,5 +1,6 @@
 import { Injectable, NotFoundException } from '@nestjs/common';
 import { session_userprofile } from '@prisma/client';
+import { EArbitraryPlannerItem } from 'src/common/entities/EArbitraryPlannerItem';
 import {
   PlannerBodyDto,
   PlannerQueryDto,
@@ -38,6 +39,14 @@ export class PlannerRepository {
     });
   }
 
+  public async getBasicPlannerById(id: number): Promise<PlannerBasic | null> {
+    return await this.prisma.planner_planner.findUnique({
+      where: {
+        id: id,
+      },
+    });
+  }
+
   public async createPlanner(
     body: PlannerBodyDto,
     arrange_order: number,
@@ -221,8 +230,11 @@ export class PlannerRepository {
 
   public async createArbitraryPlannerItem(
     planner: PlannerBasic,
-    target_item: ArbitraryPlannerItem,
-  ) {
+    target_item: Omit<
+      ArbitraryPlannerItem,
+      'id' | 'planner_id' | 'subject_department'
+    >,
+  ): Promise<EArbitraryPlannerItem.Details> {
     return await this.prisma.planner_arbitraryplanneritem.create({
       data: {
         planner_planner: {
@@ -245,6 +257,7 @@ export class PlannerRepository {
         credit: target_item.credit,
         credit_au: target_item.credit_au,
       },
+      include: EArbitraryPlannerItem.Details.include,
     });
   }
 }