PURPOSE: To enable complete separation between an electric circuit and a mechanical part by interrupting the line between a pair of light introduction paths which are so arranged that the center lines may be aligned by a light shielding plate attached to a plunger which is held up by the elastic force of a spring.

CONSTITUTION: Optical signals corresponding to electric signals through LED, etc., passing through introduction paths 4 and 6 which are so arranged that the center lines may be aligned, enter a phototransistor, etc., without being interrupted by a plunger 1 held up by the elastic force of a spring 2. On the other hand, when the plunger 1 is pressed down in the direction perpendicular to the introduction paths 4 and 6, the line between the wave guide paths 4 and 6 is interrupted by the light shielding plate 5 attached to the plunger 1 and entrance of optical signals to the phototransistor is interrupted. By this constitution, an electric circuit and a mechanical part can be separated completely, and it becomes a switch which can prevent generation of fault by short circuit and fault waves from a contact.