diff --git a/docs/1-trial-session/11-object/_samples/incrementAge/script.js b/docs/1-trial-session/11-object/_samples/incrementAge/script.js
index 218660d8a..e2bb135fe 100644
--- a/docs/1-trial-session/11-object/_samples/incrementAge/script.js
+++ b/docs/1-trial-session/11-object/_samples/incrementAge/script.js
@@ -1,5 +1,5 @@
 function incrementAge(person) {
-  person.age = person.age + 1;
+  person.age += 1;
   return person;
 }
 
diff --git a/docs/1-trial-session/11-object/index.md b/docs/1-trial-session/11-object/index.md
index a76ad31ec..96dc01758 100644
--- a/docs/1-trial-session/11-object/index.md
+++ b/docs/1-trial-session/11-object/index.md
@@ -151,7 +151,7 @@ document.write(nextYearTanaka.age); // 19 と表示されてほしい
 
 ```javascript
 function incrementAge(person) {
-  person.age = person.age + 1;
+  person.age += 1;
   return person;
 }