To remove being affected from an adjacent optical path and to improve detection accuracy by making different the modulation frequencies of both light projecting/receiving parts, making plural light emitting diodes sequentially light-emit and controlling and detecting each of them only by a corresponding light receiving means.

In the 1st and the 2nd light projecting/receiving parts 1 and 2 forming each of the optical paths, L1-PT1, L2-PT2, L3-PT3 and L4-PT4 by the plural light emitting diodes Ls and the light receiving diodes PTs, the frequencies of respective pulse modulation circuits 12 and 22 are made different, so that even if the reflected light of the its own projected light is made incident on the light receiving diodes PT, it is shut out by filter circuits 13 and 23 and is not to be outputted to a microcomputer 11 or 21. Then, the lightning of the respective light emitting diodes Ls and the light receiving timing of the corresponding light receiving diode PTs are synchronized to sequentially perform switching control to the other light emitting diode and light receiving diode. Thus, the detection of the stray light transmitted from adjacent light emitting diode, and scattering or the like can be removed, and the detection accuracy is improved.