diff --git a/lib/doc/ja/Math.lyx b/lib/doc/ja/Math.lyx
index 18d0f32947..d5982008a4 100644
--- a/lib/doc/ja/Math.lyx
+++ b/lib/doc/ja/Math.lyx
@@ -193,7 +193,7 @@ End
 \font_sf_scale 100
 \font_tt_scale 100
 \graphics default
-\default_output_format default
+\default_output_format pdf
 \output_sync 0
 \bibtex_command default
 \index_command default