To detect invasion of an object in a detection area for prescribed width by a pair of light projecting and receiving elements.

A light projecting and receiving unit 2 is provided with a pair of light projecting element 4 and light receiving element 5. The unit is provided with a plurality of conversion mirror parts 6 reflecting light and folding an optical path. An opposite unit 3 is arranged by facing the light projecting/receiving unit 2 and is provided with conversion mirror parts 13 in positions confronted with the light projecting element 4, the respective conversion mirror parts 6 and the light receiving element 5. Projected beams emitted from the light projecting element 4 reciprocate for a plurality of times between the light projecting/receiving unit 2 and the opposite unit 3 by the conversion mirror parts 6 and the conversion mirror parts 13, and they are made incident on the light receiving element 5. When the object invades between the light projecting/receiving unit 2 and the opposite unit 3, the optical path from the light projecting element 4 to the light receiving element 5 is interrupted, and the projected beam is not made incident on the light receiving element 5.