Infinite POW ball (Japanese Version)
Area 7 has a separate building on the upper right. Stand in the middle of the building and press "up", you can enter a hidden communication room. Communication will tell you to knock on the right wall 5 times. After doing it, you will hear the sound like getting 1UP, then you can use the POW ball indefinitely(press "A" "B" at the same time).
Obtain All Weapons (Japanese Version)
In Level 17's interrogation room, beat the prisoner 255 times and you'll receive all the weapons.
During the game, hold A + B and press start.
Skip the intro
Press A and B at the same time during intro.