PURPOSE: To make the measurement highly accurate and easy, by applying a coherent variable wavelength light beam, gradually varying the wavelength, thereby measuring the thickness of the transparent plate.

CONSTITUTION: The light (incident light 2) having coherent wavelength is inputted into a material to be measured 4 from a spectroscope 1. Reflected light 5 from its surface and reflected light 6 from the back surface are interfered, and the result is detected by a sensor 7. This thicknes (t) of the material to be measured is expressed by an expression t=λ×λ'/{4×(λ'-λ)}, where λ is the wavelength which corresponds to one of the highest values of the sensor outputs and λ' is the wavelength which corresponds to the lowest output value that is obtained by gradually varying the wavelength. As a result, the detecting accuracy becomes high. Since the light beam having the wavelength which can transmit through the material to be measured is selected, the measurement becomes easy.