PURPOSE: To achieve invariable stable, high-precision slit detection by detecting the existence of a slit by ≥3 sets of photodetectors which face a projector across a member moving while having the slit.

CONSTITUTION: To stop a frame-shaped jig 4, etc., having a slit 7... at a constant position, the slit 7 is detected optically. This optical detector consists of a projector 12, and ≥3 photodetectors 13W15 facing the projector 12 with the jig 4 in between and arranged in parallel in the movement direction of the jig 4. Therefore, when the jig 4 moves, the photodetectors 13W15 are all in the photodetection state and generate outputs 0... and when they are not in the photodetection state, outputs 1... are generated. Then, only the photodetector 13, photodetectors 13 and 14, only photodetector 14, photodetectors 14 and 15, only photodetector 15, etc., are placed in the photodetection state successively, and their photodetection outputs are processed digitally by a logical gate, an FF, etc., to detect the slit without malfunction even at a boundary part, etc., thus achieving invariably stable, high-precision slit detection.