PURPOSE: To easily and correctly measure a thin film without overlapping interference fringes on a photographing element by storing a waveform of a reference interference fringe beforehand and subtracting the waveform of the reference interference fringe from a waveform of a measured interference fringe.

CONSTITUTION: An illumination light from a light-emitting device is reflected from a projecting device 1 to a half mirror 2 and cast on an object to be measured via an objective lens 3. The light reflected by the object enters a Wollaston prism 6, is divided into a P polarization wave and an S polarization wave and made incident on a CCD camera 9. After an output waveform from the camera 9 is amplified, the waveform is shaped by filtering and trimmed at an image take block 12. Waveforms of signals are averaged in a computer 13, from which a waveform resulting from the measurement of the object without a film formed, that is, a waveform having only a reference interference fringe is subtracted. Then, peaks of waveforms A, B of interference fringes are obtained and the distance of the peaks is calculated, thereby, the thickness of the film is obtained.