diff --git a/.gitignore b/.gitignore
index 60e229d..6649241 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,4 +2,5 @@
 /fukushima
 /assembly_result
 /mutants.out
+/mutants.out.old
 /.vs
\ No newline at end of file