Abstract: A photoelectric detecting device reads the alignment mark of a mask and the alignment mark of a wafer with high accuracy. In this device, a photoelectric detecting system for detecting the light reflected from the mask and a photoelectric detecting system for detecting the light reflected from the wafer are provided separately from each other to read the two marks with high accuracy. To distinguish between the light reflected from the mask and the light reflected from the wafer, a .lambda./4 plate is positioned between the mask and the wafer. The device also uses a polarized light beam to illuminate the mask and wafer.