diff --git a/src/app/app.component.html b/src/app/app.component.html
index 0ede04b..6e3817a 100644
--- a/src/app/app.component.html
+++ b/src/app/app.component.html
@@ -1,10 +1,7 @@
-<div [formGroup]="form">
-  <div formArrayName="Tasks" *ngFor="let control of tasks.controls; let index = index">
-    <ng-container [formGroupName]="index">
-      <input type="text" placeholder="主旨" formControlName="Subject" />
-      <input type="text" placeholder="內容" formControlName="Content" />
-    </ng-container>
-  </div>
-  <span *ngIf="tasks.hasError('cannotEmpty')">至少需要一筆工作事項</span>
+<div>
+  單價：<input type="text" [formControl]="formControl" />
+  <span *ngIf="formControl.hasError('decimal')">
+    單價應為 {{ formControl.errors?.decimal.digitLength }} 位整數位與
+    {{ formControl.errors?.decimal.scaleLength }} 位整數位的數值
+  </span>
 </div>
-<pre>{{ tasks.errors | json }}</pre>
diff --git a/src/app/app.component.ts b/src/app/app.component.ts
index 36232db..7f3814c 100644
--- a/src/app/app.component.ts
+++ b/src/app/app.component.ts
@@ -1,23 +1,14 @@
 import { Component } from '@angular/core';
-import { FormArray, FormBuilder } from '@angular/forms';
-
-import { arrayCannotEmpty } from './validators/array-cannot-empty.validator';
+import { FormControl } from '@angular/forms';
+import { decimalValidator } from './validators/decimal.validator';
 
 @Component({
   selector: 'my-app',
   templateUrl: './app.component.html',
-  styleUrls: ['./app.component.css'],
+  styleUrls: [ './app.component.css' ]
 })
-export class AppComponent {
-  readonly form = this.fb.group({
-    tasks: this.fb.array([], {
-      validators: [arrayCannotEmpty]?
-    }),
+export class AppComponent  {
+  readonly formControl = new FormControl(0, {
+    validators: [decimalValidator(2, 1)]
   });
-
-  get tasks(): FormArray {
-    return this.form.get('tasks') as FormArray;
-  }
-
-  constructor(private fb: FormBuilder) {}
 }
diff --git a/src/app/app.module.ts b/src/app/app.module.ts
index 927c20c..5e6e85e 100644
--- a/src/app/app.module.ts
+++ b/src/app/app.module.ts
@@ -1,6 +1,6 @@
 import { NgModule } from '@angular/core';
-import {  ReactiveFormsModule } from '@angular/forms';
 import { BrowserModule } from '@angular/platform-browser';
+import { ReactiveFormsModule } from '@angular/forms';
 
 import { AppComponent } from './app.component';
 
diff --git a/src/app/validators/array-cannot-empty.validator.ts b/src/app/validators/array-cannot-empty.validator.ts
deleted file mode 100644
index ad9ada6..0000000
--- a/src/app/validators/array-cannot-empty.validator.ts
+++ /dev/null
@@ -1,8 +0,0 @@
-import { FormArray, ValidationErrors } from '@angular/forms';
-
-export function arrayCannotEmpty(formArray: FormArray): ValidationErrors | null {
-  if (formArray.length === 0) {
-    return { cannotEmpty: true };
-  }
-  return null;
-}
diff --git a/src/app/validators/decimal.validator.ts b/src/app/validators/decimal.validator.ts
new file mode 100644
index 0000000..e43fe77
--- /dev/null
+++ b/src/app/validators/decimal.validator.ts
@@ -0,0 +1,23 @@
+import { AbstractControl, ValidationErrors, ValidatorFn } from '@angular/forms';
+
+export function decimalValidator(
+  digitLength: number,
+  scaleLength: number
+): ValidatorFn {
+  return (control: AbstractControl): ValidationErrors | null => {
+    if (
+      control.value === undefined ||
+      control.value === null ||
+      control.value === ''
+    ) {
+      return null;
+    }
+
+    const regular = new RegExp(
+      `^\\d{1,${digitLength}}(\\.\\d{0,${scaleLength}})?$`
+    );
+    return regular.test(control.value)
+      ? null
+      : { decimal: { digitLength, scaleLength } };
+  };
+}