After beating the game, when the Kanji "The End" appears, press Up, Up, Down, Down, Left, Right, Left, Right, B, A, B, A, A, B, A, B, Right, Left, Right, Left, Down, Down, Up & Up.
At the map menu screen, enter the second option (which gives you your current password), wait until the password is displayed and press Up, Up, Down, Down, Left, Right, Left, Right, B & A. If done right, a sound will be heard. Now start your game and you will be invincible. *Note* this can only be done after completing the first stage. To disable Invincibility, do the cheat a second time.
At the very first briefing screen, after selecting start or continue and before the map screen appears, press and hold A, B, Select & Start. If done right, the sound test should appear.
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