An illuminating push button switch having a mistake-proof design to avoid improper installation of a light emitting diode (LED) consists essentially of a body portion (2), a cap portion (1) attached above said body portion (2), and a LED (6) inserted into the body portion (2) from the bottom. Said push button switch is characterized in that said body portion (2) incorporates a light cap (22), and a slot (23) for receiving the LED (6). A frame portion (30) is reciprocating along the light cap (22) for activating/deactivating the switch. Said mistake-proof design is provided at an inner wall of the slot (23).