diff --git a/apply_factor.py b/apply_factor.py
index 1a66365f..2b3613c0 100755
--- a/apply_factor.py
+++ b/apply_factor.py
@@ -87,7 +87,7 @@
 
     grid = utils.save_image(
         torch.cat([img1, img, img2], 0),
-        f"{args.out_prefix}_index-{args.index}_degree-{args.degree}.png",
+        f"{args.out_prefix}_index-{args.index:02d}_degree-{args.degree}.png",
         normalize=True,
         range=(-1, 1),
         nrow=args.n_sample,