With a film thickness measuring apparatus of the present invention, a substrate having a transparent film formed on its front surface is placed on a placement unit. A half mirror divides light from a light source such that divided light beams are emitted to the front surface of the substrate and to a reference plane, and overlays reflected light from the front surface of the substrate and reflected light from the reference plane on each other to form interfering light. The interfering light is imaged by an imager. Based on the imaging result, the film thickness of the transparent film is calculated by an arithmetic unit. An optical filter with which an intensity spectrum of transmitted light exhibits a plurality of peaks, the optical filter is disposed between the light source and the half mirror.