diff --git a/README.md b/README.md
index 52a39ae..791f736 100644
--- a/README.md
+++ b/README.md
@@ -75,7 +75,7 @@ module.exports = withPlugins([
 
 ### When controlling by IP
 
-Control by IP of access source (x-forwarded-for).  
+Control by IP of request source (x-forwarded-for).  
 Specify the IPs to be allowed as a string or an array of strings, which can be written in CIDR format.
 
 ```js
@@ -89,12 +89,19 @@ const withFortress = require('next-fortress')({
     {
       inspectBy: 'ip',
       ips: ['11.22.33.0/24', '111.222.0.0/16'], // It can be written in CIDR format.
+      failSafe: true, // See description below.
       // ...controlMode
     }
   ]
 })
 ```
 
+It uses `x-forwarded-for` to get the IP of the requestor. Therefore, it is not possible to control by IP in the development environment.  
+You can use `failSafe` to control whether the request is controlled or passed through in the case that such an IP could not be gotten. 
+- `failSafe?: boolean`: The default value is `process.env.NODE_ENV === 'production'`.
+    - `true`: If it fails to get the IP, it must control the request according to the control mode.
+    - `false`: If it fails to get the IP, the request will be passed through.
+
 #### Redirect
 ```js
 const withFortress = require('next-fortress')({
@@ -113,7 +120,7 @@ const withFortress = require('next-fortress')({
 })
 ```
 
-#### Access Block
+#### Request Block
 ```js
 const withFortress = require('next-fortress')({
   forts: [
@@ -173,7 +180,7 @@ const withFortress = require('next-fortress')({
 
 **Control Mode** 
 - `redirect`: See [Redirect](#redirect)
-- `block`: See [Access Block](#access-block)
+- `block`: See [Request Block](#request-block)
 - `rewrite`: See [Rewrite](#rewrite)
 
 Put useFortressWithFirebase in your page/_app.jsx.  
@@ -211,7 +218,7 @@ const withFortress = require('next-fortress')({
 
 **Control Mode** 
 - `redirect`: See [Redirect](#redirect)
-- `block`: See [Access Block](#access-block)
+- `block`: See [Request Block](#request-block)
 - `rewrite`: See [Rewrite](#rewrite)
 
 Add `ssr: true` to the client side `Amplify.configure`.
@@ -243,7 +250,7 @@ const withFortress = require('next-fortress')({
 
 **Control Mode** 
 - `redirect`: See [Redirect](#redirect)
-- `block`: See [Access Block](#access-block)
+- `block`: See [Request Block](#request-block)
 - `rewrite`: See [Rewrite](#rewrite)
 
 ---
@@ -307,7 +314,7 @@ const withFortress = require('next-fortress')({
 
 ## :construction: Caution
 
-**Please do not use `next/link` to generate links to pages subject to access control.**
+**Please do not use `next/link` to generate links to pages you want to control access to.**
 
 The link tag generated by `next/link` will request the target page for a js file when clicked. Unfortunately, it is not possible to control access to that URL.