PURPOSE: To clearly recognize at which of positions the number of fringes of an interference fringe images is counted by forming an opening at a screen provided in parallel beams in which a reference wave and a wave to be detected are superposed and inputting only passed beam to a sensor.

CONSTITUTION: Reflected lights to be interfered on a reference surface 6a and a surface 7a to be detected are reflected on a surface 4c, and interference fringe images are focused on sensors 13, 14. In this case, the reference wave of the surface 6a and the wave to be detected of the surface 7a are so formed to be superposed tap become parallel beams that openings C1-Cm are alternately formed correspondingly to bright and dark parts on a straight line to be superposed with interference fringes at an interval (d) separated at nπ/2 (n is natural number) of a phase on the fringes at a screen 16 provided at a position to become the parallel beams. Thus, spot images C1'-Cm' are focused only at parts passed through the openings C1-Cm at a focused surface 17. A sensor 13 is disposed on the surface 17, number of inversions of element outputs to be superposed with the images c1'-Cm' on a line sensor is counted to calculate a displacement of the surface 7a. The openings C1-Cm are formed to clearly recognize at which position of the fringes is counted.