PURPOSE: To obtain a method and an apparatus which enables accurate measurement of a minute displacement value in the direction of an optical axis necessary for determining a shape and surface accuracy of a surface to be measured.

CONSTITUTION: Coherent light from the same light source 1 is projected on a surface 7a to be inspected and a control surface 6a as reference to produce an interference fringe image 11 moving a specimen with a translation base 8. First type sensors Sa1 and Sa2 are provided at observing points P1 and P2 with a phase difference of the interference fringe images separated almost by π/2 and a second type sensor Sb larger than a fringe interval of the interference fringes is provided at another location. Sinusoidal outputs from the first type sensors are converted to square waves and pulse waves sequentially. At this point. an average intensity of the interference fringes is determined with the second type sensors and a conversion to a square wave is performed with the results as threshold level. The determination of the average intensity enables accurate measurement even when the contrast of the interference fringe images is low.