To realize accurate measurements with a fluorescent X-ray film thickness gauge characterized in that it makes measurements non-destructively and without contact.

The fluorescent X-ray film thickness gauge comprises a source of X-rays; a collimator for focusing primary X-rays; a sample observing optical system for positioning and observing microscopic portions; and juxtaposed detectors for fluorescent X-rays generated from a sample as film thickness measuring means, with one of the detectors being a sensor excellent in energy resolution although low in counting efficiency, for counting low energies, and the other being a sensor excellent in counting efficiency although poor in energy resolution, for counting high energies. An individual linear amplifier and a pulse height analyzer are provided behind the rear preamplifier of the detectors and pulse height data are processed by a common control and computing part as spectra for qualitative and quantitative analyses.