A position detecting apparatus usable for aligning mask and a semiconductor wafer, wherein a laser beam produced by a semiconductor laser is projected through a predetermined optical system to alignment marks formed on the mask and the wafer, and the light reflected by the marks are detected by an accumulation type sensor to produce an electrical signal, from which the relative positional relation between the mask and the wafer are detected on the basis of the electrical signal. To obtain proper mark signals, the quantity of light incident on the accumulation sensor is controlled. In this apparatus, the beam emitting strength of the semiconductor laser is made constant, and the control of thee amount of light incident on the accumulation sensor is effected by controlling the operation period of the semiconductor laser. In addition, the actuation timing of the semiconductor laser is advanced from the accumulation start of the accumulation type sensor by the time required for the semiconductor laser to be thermally stabilized after its actuation. The mark detection signal produced by the accumulation sensor is precise.