These codes are for the Japanese version of the game. (This is the Japanese release of Contra III: The Alien Wars)

Level Select
Press Left, Down, Down/Left, Left, Start at the player selection screen. A hum will confirm correct code entry.

All Weapons
Press Right, Down, Down/Right, Right, Start at the player selection screen. A death scream will confirm correct code entry.

Screen Spin
Briefly press L or R once, then hold the button in levels two or four. The screen will spin rapidly.

In-Game Reset
Hold L + R + Start and press Select during game play to return to the Title Screen.

Sound Test
Rapidly tap X while quickly pressing Down, Right on the Title Screen.

Have 30 Lives
Quickly press Down, Down/Right, Right, Start at the Title Screen.