PURPOSE: To provide an apparatus for measuring a thickness of a painted film without touching the film which is utilizable when the painted surface is dry and when the painted surface is wet.

CONSTITUTION: Reference data indicating a relation of a phase difference between an intensity of an acoustic signal (y) subsequent to an opto-acoustic effect and a laser light (x) and a thickness of a painted film are formed for every kind of paints of different colors and stored in a memory means 11. When the pulse-like laser light (x) is cast from an illuminating means 5 to a painted surface of an object (a), the acoustic signal (y) is generated from a painted film (b) due to an opto-acoustic effect, which is detected by a microphone 7. The detected acoustic signal (y) is sent to an analyzing means 9, where a phase difference of an intensity of the signal and the laser light (x) is analyzed. An analyzing signal is fed to a comparing means 12 and compared with the reference data of a corresponding paint read from the memory means 11, whereby a thickness of the painted film is determined in accordance with a value of tone signal. The determined value is indicated by a display means 13 as a measuring value of the film thickness.