prior to a periphery exposing operation , a light emitting unit and a light receiving unit are retracted to a position separate from a wafer , and a calibrating operation is conducted in the retracted position utilizing a light shield plate different from the wafer to be exposed , thereby obtaining a servo control reference signal for the exposure . at the exposure of the periphery portion of the wafer , the exposure width is controlled according to the reference signal . it is therefore rendered possible to achieve exact peripheral exposure , including the intensity and intensity distribution of the exposing light beam immediately before the irradiation of the peripheral portion of the wafer .