diff --git a/language/interpreter.cc b/language/interpreter.cc
index 8b29ac0..8f4a94c 100644
--- a/language/interpreter.cc
+++ b/language/interpreter.cc
@@ -140,6 +140,7 @@ private:
                 case '-': ret -= val; break;
                 case '*': ret *= val; break;
                 case '/': ret /= val; break;
+                case '%': ret %= val; break;
             }
         }
         return Value(ret);
diff --git a/language/parser.cc b/language/parser.cc
index 25bc3db..9f155f2 100644
--- a/language/parser.cc
+++ b/language/parser.cc
@@ -26,7 +26,7 @@ static auto g_grammar = R"(
 
     CONDITION_OPERATOR  <-  < ('==' / '!=' / '<=' / '<' / '>=' / '>') > _
     TERM_OPERATOR       <-  < [-+] > _
-    FACTOR_OPERATOR     <-  < [*/] > _
+    FACTOR_OPERATOR     <-  < [*/%] > _
     IDENTIFIER          <-  < [a-zA-Z_]+ > _
 
     NUMBER              <-  < [0-9]+ > _