At the title screen, press C-Up, C-Up, C-Down, C-Down, C-Left, C-Right, C-Left, C-Right, B, A, then hold Z and press Start.

At the title screen, press Up, L, Up, L, Down, L, Down, L, Left, R, Right, R, Left, R, Right, R, B, A, then hold Z and press Start.
Note: This may prevent you from saving your game in the American version. (It works fine in J-League Perfect Striker.)