To always accurately and quickly measure the thickness of a plate being a measuring object, irrespective of the material quality, hardness, and the area of the plate.

The thickness of the transparent plate is calculated on the basis of the relation between thickness and spherical aberration in a reference basal plate, by using the spherical aberration component of an interference fringe to be obtained by light reflected by a reference face, and light reflected by the rear face of the transparent plate arranged at a predetermined position after permeating the reference face and having permeated the reference face again.