diff --git a/.github/workflows/verify-on-macos.yml b/.github/workflows/verify-on-macos.yml
index 469120b..19ab8ee 100644
--- a/.github/workflows/verify-on-macos.yml
+++ b/.github/workflows/verify-on-macos.yml
@@ -13,7 +13,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
     - name: print created files
       run: ls -l ~/.ssh
     - name: git clone through SSH
@@ -28,7 +28,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
         name: ssh_key_name # optional
         config: | # optional
           Host github
diff --git a/.github/workflows/verify-on-ubuntu.yml b/.github/workflows/verify-on-ubuntu.yml
index e6548e1..8b165c5 100644
--- a/.github/workflows/verify-on-ubuntu.yml
+++ b/.github/workflows/verify-on-ubuntu.yml
@@ -13,7 +13,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
     - name: print created files
       run: ls -l ~/.ssh
     - name: git clone through SSH
@@ -28,7 +28,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
         name: ssh_key_name # optional
         config: | # optional
           Host github
diff --git a/.github/workflows/verify-on-ubuntu1604.yml b/.github/workflows/verify-on-ubuntu1604.yml
index b3cce08..fdd7d67 100644
--- a/.github/workflows/verify-on-ubuntu1604.yml
+++ b/.github/workflows/verify-on-ubuntu1604.yml
@@ -13,7 +13,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
     - name: print created files
       run: ls -l ~/.ssh
     - name: git clone through SSH
@@ -28,7 +28,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
         name: ssh_key_name # optional
         config: | # optional
           Host github
diff --git a/.github/workflows/verify-on-windows.yml b/.github/workflows/verify-on-windows.yml
index 003466f..6e58f05 100644
--- a/.github/workflows/verify-on-windows.yml
+++ b/.github/workflows/verify-on-windows.yml
@@ -13,7 +13,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
     - name: print created files
       run: ls ~/.ssh
     - name: git clone through SSH
@@ -28,7 +28,7 @@ jobs:
       uses: ./.
       with:
         key: ${{ secrets.SSH_KEY }}
-        known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+        known_hosts: ${{ secrets.KNOWN_HOSTS }}
         name: ssh_key_name # optional
         config: | # optional
           Host github
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 55bebe3..79f9ea6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 * rename `private-key` to `key`
 * rename `known-hosts` to `known_hosts`
+* make `known_hosts` required
 
 ## [1.6.4] - 2020-01-27
 
diff --git a/README.md b/README.md
index ac298dc..acfb9d3 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@ steps:
   with:
     key: ${{ secrets.SSH_KEY }}
     name: id_rsa # optional
-    known_hosts: ${{ secrets.KNOWN_HOSTS }} # optional
+    known_hosts: ${{ secrets.KNOWN_HOSTS }}
     config: ${{ secrets.CONFIG }} # ssh_config; optional
 - name: rsync over ssh
   run: rsync ./foo/ user@remote:bar/
@@ -84,9 +84,7 @@ Check belows:
     * OPENSSH format (key begins with `-----BEGIN OPENSSH PRIVATE KEY-----`) may not work.
     * Use PEM format (begins with `-----BEGIN RSA PRIVATE KEY-----`).
 * `Host key verification failed.`:
-    * Set `known_hosts` option or use `ssh -o StrictHostKeyChecking=no`.
-    * The former is **HIGHLY** recommended for security reason.
-    * I'm planning to make `known_hosts` required in v2.
+    * Set `known_hosts` option correctly (use `ssh-keyscan` command).
 
 ### How do I use encrypted SSH key?
 
diff --git a/action.yml b/action.yml
index aef5b4e..bf35c34 100644
--- a/action.yml
+++ b/action.yml
@@ -15,7 +15,7 @@ inputs:
     default: "id_rsa"
   known_hosts:
     description: "public keys of SSH servers"
-    required: false
+    required: true
     default: ""
   config:
     description: "SSH config"
diff --git a/lib/main.js b/lib/main.js
index b97cccb..76eeb65 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -28,7 +28,9 @@ function main() {
             },
             {
                 name: "known_hosts",
-                contents: prependLf(core.getInput("known_hosts")),
+                contents: prependLf(core.getInput("known_hosts", {
+                    required: true,
+                })),
                 options: {
                     mode: 0o644,
                     flag: "a",
diff --git a/lib/main.js.map b/lib/main.js.map
index 43bb0e4..3fa68cc 100644
--- a/lib/main.js.map
+++ b/lib/main.js.map
@@ -1 +1 @@
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\ No newline at end of file
+{"version":3,"file":"main.js","sourceRoot":"","sources":["../src/main.ts"],"names":[],"mappings":";;;;;;;;;AAAA,uCAAyB;AACzB,2CAA6B;AAE7B,oDAAsC;AAStC;;GAEG;AACH,SAAS,IAAI;IAEZ,IACA;QACC,MAAM,KAAK,GAAe;YACzB;gBACC,IAAI,EAAE,IAAI,CAAC,QAAQ,CAAC,MAAM,CAAC;gBAC3B,QAAQ,EAAE,IAAI,CAAC,QAAQ,CAAC,KAAK,EAAE;oBAC9B,QAAQ,EAAE,IAAI;iBACd,CAAC;gBACF,OAAO,EAAE;oBACR,IAAI,EAAE,KAAK;oBACX,IAAI,EAAE,IAAI;iBACV;aACD;YACD;gBACC,IAAI,EAAE,aAAa;gBACnB,QAAQ,EAAE,SAAS,CAAC,IAAI,CAAC,QAAQ,CAAC,aAAa,EAAE;oBAChD,QAAQ,EAAE,IAAI;iBACd,CAAC,CAAC;gBACH,OAAO,EAAE;oBACR,IAAI,EAAE,KAAK;oBACX,IAAI,EAAE,GAAG;iBACT;aACD;YACD;gBACC,IAAI,EAAE,QAAQ;gBACd,QAAQ,EAAE,SAAS,CAAC,IAAI,CAAC,QAAQ,CAAC,QAAQ,CAAC,CAAC;gBAC5C,OAAO,EAAE;oBACR,IAAI,EAAE,KAAK;oBACX,IAAI,EAAE,GAAG;iBACT;aACD;SACD,CAAC;QAEF,0BAA0B;QAC1B,MAAM,IAAI,GAAG,gBAAgB,EAAE,CAAC;QAChC,MAAM,OAAO,GAAG,IAAI,CAAC,OAAO,CAAC,IAAI,EAAE,MAAM,CAAC,CAAC;QAC3C,EAAE,CAAC,SAAS,CAAC,OAAO,EAAE;YACrB,SAAS,EAAE,IAAI;YACf,IAAI,EAAE,KAAK;SACX,CAAC,CAAC;QAEH,eAAe;QACf,KAAI,MAAM,IAAI,IAAI,KAAK,EACvB;YACC,MAAM,QAAQ,GAAG,IAAI,CAAC,IAAI,CAAC,OAAO,EAAE,IAAI,CAAC,IAAI,CAAC,CAAC;YAC/C,EAAE,CAAC,aAAa,CAAC,QAAQ,EAAE,IAAI,CAAC,QAAQ,EAAE,IAAI,CAAC,OAAO,CAAC,CAAC;SACxD;QAED,OAAO,CAAC,GAAG,CAAC,8BAA8B,OAAO,gBAAgB,CAAC,CAAC;KACnE;IACD,OAAM,GAAG,EACT;QACC,IAAI,CAAC,SAAS,CAAC,GAAG,CAAC,OAAO,CAAC,CAAC;KAC5B;AACF,CAAC;AAED;;;GAGG;AACH,SAAS,gBAAgB;IAExB,MAAM,OAAO,GAAG,UAAU,EAAE,CAAC;IAC7B,MAAM,IAAI,GAAG,OAAO,CAAC,GAAG,CAAC,OAAO,CAAC,CAAC;IAClC,IAAG,IAAI,KAAK,SAAS,EACrB;QACC,MAAM,KAAK,CAAC,GAAG,OAAO,iBAAiB,CAAC,CAAC;KACzC;IAED,OAAO,IAAI,CAAC;AACb,CAAC;AAED;;;GAGG;AACH,SAAS,UAAU;IAElB,IAAG,OAAO,CAAC,QAAQ,KAAK,OAAO,EAC/B;QACC,UAAU;QACV,OAAO,aAAa,CAAC;KACrB;IAED,gBAAgB;IAChB,OAAO,MAAM,CAAC;AACf,CAAC;AAED;;;;GAIG;AACH,SAAS,SAAS,CAAC,KAAa;IAE/B,IAAG,KAAK,CAAC,MAAM,KAAK,CAAC,EACrB;QACC,sBAAsB;QACtB,OAAO,EAAE,CAAC;KACV;IAED,OAAO,KAAK,KAAK,EAAE,CAAC;AACrB,CAAC;AAED,IAAI,EAAE,CAAC"}
\ No newline at end of file
diff --git a/src/main.ts b/src/main.ts
index b3aa60c..7a191cb 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -30,7 +30,9 @@ function main(): void
 			},
 			{
 				name: "known_hosts",
-				contents: prependLf(core.getInput("known_hosts")),
+				contents: prependLf(core.getInput("known_hosts", {
+					required: true,
+				})),
 				options: {
 					mode: 0o644,
 					flag: "a",