A light refracting element formed in parallel plate shape is attached to a support shaft of a mirror holder, a semiconductor laser and a PSD are disposed at positions between which the light refracting element is sandwiched. The light refracting element is rotated by rotation of the mirror holder, and whereby a laser beam irradiation position is changed on a light acceptance surface of PSD. The laser beam irradiation position on a light acceptance surface corresponds to the mirror rotation position, so that the mirror rotation position and a laser beam scanning position in a target area can be detected based on an output from the PSD.