Now, when the beams LB1P and LB2P enter the wafer mark WM at the intersecting angles 2??W, one-dimensional interference fringes are produced at a pitch PF (defined as PF =PW /2 in the present embodiment) which is 1/N times the pitch PW of the wafer mark WM (where N is a natural number) in an arbitrary plane perpendicular to the optical axis AX in the spatial area where the beam LB1P and LB2P are i