An optical film thickness meter capable of measuring an optical film thickness and spectroscopic characteristics highly accurately, and a thin film forming apparatus with the optical film thickness meter are provided. The optical film thickness meter includes a light projector, a light receiver, a monochromator, and a reflection mirror having a reflection surface substantially perpendicularly to the optical axis of measurement light on the side opposite to an actual substrate. The actual substrate is disposed having a predetermined angle to the optical axis. The measurement light passes through the actual substrate twice, whereby a change in transmissivity can be increased, and control accuracy of thickness measurement is improved. Measurement errors caused by a difference in transmission positions is prevented. Since the measurement light which has not passed through the measurement substrate twice is not detected by the light receiver, the optical film thickness and spectroscopic characteristics is measured highly accurately.