An apparatus for measuring a thickness of an object transparent to light utilizing an interferometric method includes a light source for generating a coherent light beam to which an object to be measured is transparent, an illumination unit for radiating onto the object the incident light beam as scanned over a range of angle of incidence varying from a predetermined angle of incidence .theta..sub.A to another predetermined angle of incidence .theta..sub.B, and a photosensor unit for detecting light intensity changes resulting from changing of the optical path difference between two light beams reflected by the upper and lower surfaces of the object. A count circuit receives an output signal from the photosensor unit and counts the difference between an order of interference fringes obtained for one scan of incident light beam having the predetermined angle of incidence .theta..sub.A to that having the other predetermined angle of incidence .theta..sub.B, and a calculating circuit converts the output from the count circuit into a value corresponding to the thickness of the object.