A light illumination membrane switch having a configuration with a reduced size, capable of realizing an effective light illumination of the keys and a highly reliable operation even under the high humidity environment. The switch has a light source section using an LED bare chip element for emitting lights to illuminate the key section which is directly mounted on the electrode circuit member of the printed circuit board. The switch may includes a translucent diffuser member having a cavity region for directly transmitting the lights emitted from the light source section to the key section. The switch may includes a translucent diffuser member having a side wall portion defining an opening such that the translucent diffuser member and the printed circuit board are assembled together by inserting the printed circuit board into the opening defined by the side wall portion. The switch may includes a translucent diffuser member having reflective coating layer formed thereon for reflecting the lights emitted from the light source section into the translucent diffuser member.