diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 34bc1a93..e80d7cc3 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -1,5 +1,5 @@
 find_package(FLEX REQUIRED)
-flex_target(scan src/scan.l src/lex.yy.c COMPILE_FLAGS --header-file="src/lex.yy.h")
+flex_target(scan src/scan.l ${PROJECT_SOURCE_DIR}/src/lex.yy.c COMPILE_FLAGS --header-file="src/lex.yy.h")
 
 add_library(picrin SHARED
   src/blob.c src/bool.c src/char.c src/codegen.c src/cont.c src/error.c