PURPOSE: To easily obtain an optical screen, and to simplify monitoring, by curving a light beam from one light emitting and photodetecting device into a prescribed direction by a specular surface of a rotary body, and making it scan.

CONSTITUTION: A light beam being in parallel with a rotary shaft 1, which has been emitted from a light emitting and photodetecting device 13 hits against a specular surface 10 in a projecting part 9 of a reflection element body 8, its direction is changed by 90°, and it is reflected by a reflector 14. Subsequently, the reflection element body 8 is rotated by rotation of the rotary shaft 1, and when the specular surface 10 in its projecting part 9 is shifted from the optical axis of the light emitting and photodetecting device 13, a light beam from the light emitting and photodetecting device 13 hits against the specular surface 10 of the following element 8, an optical screen is formed like a screen in a space between a rotary body 7 and the reflector 14, and by interception of this optical screen, entrance of a hand, etc. are detected exactly for preventing danger in a press machine, etc.