PURPOSE: To achieve focusing without moving the optical element in a rear surface detecting optical system by optically correcting error due to shift of focus in a rear surface detecting optical system caused by fluctuation of the thickness of wafer.

CONSTITUTION: Focus of a rear surface position detecting optical system 27 shifts every time when the thickness of wafer 3 varies, and thereby focusing of a demagnification lens 22 onto the wafer 3 is performed through gap sensors 36, 23. The wafer 3 is mounted on XYZθ tables 28, 30, 31 being driven through a drive unit 33. Position of a reticle 19 is measured through a position detecting optical system 21 and a signal is delivered to a system control unit 37. Position of the wafer 3 is measured through the rear surface detecting optical system 27 and a signal is delivered to the system control unit 37. The system control unit 37 calculates relative positional shift and commands the XYZθ table drive unit 33 to move the wafer 3 to a desired position. Consequently, mechanical scanning is not required in the optical correction.