The command setxkbmap allows us to modify any key mapppings in user X session. However each time the input method is changed, the modified states will be reset. Since I have no clue to solve this problem, I decided to rewrite the global symbol file at my own risk.
$ cp /usr/share/X11/xkb/symbols/jp /path/to/backup/jp.default
$ patch -u /usr/share/X11/xkb/symbols/jp < js-ansi.patch
Before editing xkb resources, I highly recommend that you save the current keymap. Just save the original keymap and restore it.