diff --git a/README.md b/README.md
index 74dc076..b726187 100644
--- a/README.md
+++ b/README.md
@@ -39,6 +39,8 @@ plugins:
     opt: paths=source_relative
 ```
 
+If you want to generate TypeScript validation code, please see [`@cybozu/protoc-gen-es-cybozu-validate`](es/packages/protoc-gen-es-cybozu-validate/README.md).
+
 ## API documentation
 
 Visit https://buf.build/cybozu/protobuf
diff --git a/es/packages/protobuf/README.md b/es/packages/protobuf/README.md
index fc61ef5..82f8b2a 100644
--- a/es/packages/protobuf/README.md
+++ b/es/packages/protobuf/README.md
@@ -1 +1,11 @@
 # @cybozu/protobuf
+
+A library that provides JavaScript code generated from the Protocol Buffers schema provided by this repository.
+
+**This package is not intended to be used by external applications as a library.** It is used as an internal library for creating protoc plugins such as [`protoc-gen-es-cybozu-validate`](../protoc-gen-es-cybozu-validate/).
+
+## Inatall
+
+```
+npm i @cybozu/protobuf
+```
diff --git a/es/packages/protoc-gen-es-cybozu-validate/README.md b/es/packages/protoc-gen-es-cybozu-validate/README.md
index a6f1cf9..9d8f2a7 100644
--- a/es/packages/protoc-gen-es-cybozu-validate/README.md
+++ b/es/packages/protoc-gen-es-cybozu-validate/README.md
@@ -1 +1,82 @@
 # @cybozu/protoc-gen-es-cybozu-validate
+
+A protoc plugin that generates validators for TypeScript from the custom fields defined in [`cybozu.validate`](../../../README.md#cybozuvalidate).
+
+## Inatall
+
+```
+npm i @cybozu/protoc-gen-es-cybozu-validate
+```
+
+## Usage
+
+The following is an example `buf.gen.yaml` to generate validation code:
+
+```yaml
+version: v1
+plugins:
+  - plugin: es
+    out: .
+    opt:
+      - target=ts
+  - plugin: es-cybozu-validate
+    out: .
+```
+
+## Examples
+
+`protoc-gen-es-cybozu-validate` takes the following message type as input:
+
+```proto
+message Scalars {
+  float float = 1 [(cybozu.validate.rules).float = {lt: 3.2}];
+  double double = 2 [(cybozu.validate.rules).double = {gt: 3.2}];
+}
+```
+
+And outputs the following TypeScript code:
+
+```ts
+export const ScalarsValidators: {
+  /**
+   * @generated from field: float float = 1;
+   */
+  validateFloat: (value: unknown) => asserts value is Scalars["float"];
+  /**
+   * @generated from field: double double = 2;
+   */
+  validateDouble: (value: unknown) => asserts value is Scalars["double"];
+} = {
+  /**
+   * @generated from field: float float = 1;
+   */
+  validateFloat(value) {
+    if (typeof value !== "number") {
+      throw new CybozuValidateTypeError("number", typeof value);
+    }
+    if (value >= 3.200000047683716) {
+      throw new CybozuValidateNumberRuleError({ lt: 3.200000047683716 }, value);
+    }
+  },
+  /**
+   * @generated from field: double double = 2;
+   */
+  validateDouble(value) {
+    if (typeof value !== "number") {
+      throw new CybozuValidateTypeError("number", typeof value);
+    }
+    if (value <= 3.2) {
+      throw new CybozuValidateNumberRuleError({ gt: 3.2 }, value);
+    }
+  },
+};
+```
+
+This is designed to provide validation functions for each field, not for each message type, as it is intended for frontend form validation.
+
+## Limitation
+
+`protoc-gen-es-cybozu-validate` does not support following features:
+
+- RegExp validation
+- UNICODE and PRECIS normalization