An optical film thickness measuring device, enabling direct measurement of a film thickness of a product in real time accurately without a monitor substrate, includes: a projector, a light receiver, inner beam splitters disposed in a base substrate holder to reflect a measurement beam to a base substrate, an inner optical reflector that totally reflects a measurement beam from the closest inner beam splitter, external beam splitters the measurement beam from the inner beam splitters toward the light receiver, and an outer optical reflector that reflects the measurement beam from the optical reflector toward the light receiver. The measurement beam reflected by the inner beam splitters and the inner optical reflector is passed through the base substrate and then reflected by the external beam splitters and the outer optical reflector to be guided to the light receiver, so that the measurement beam is received by the light receiver.