PURPOSE: To obtain a highly reliable reflection type photosensor for detecting wafer no matter what the thickness of a film is on the rear of a wafer by installing a flood-lighting means which applies light having two kinds or more of wavelength to a wafer.

CONSTITUTION: A V-shaped groove is provided in the center of a sensor-bracket 1 and two holes are made in both slanting faces of the groove so that respective holes are made to be perpendicular to respective slanting faces. Light emitting elements 2a and 2b are disposed in each hole of one side face of the groove and photodetectors 3a and 3b are disposed in each hole of the other side face of the groove. The light emitting elements 2a and 2b project light containing different wavelengths respectively. At least either of rays of reflecting light out of the rays of light projected by the light emitting elements 2a and 2b is detected by each photodetector 3a or 3b and detecting signals are outputted from each photodetector. Further, after being amplified, the signals are outputted onto a controller. As its sensor always performs stable operations no matter what the thickness of a film is on the rear of a wafer, improper matters such as detection failure, deviation of detecting positions of the wafer and the like are avoided and highly reliable detection of the wafer is obtained.