To easily and accurately measure coating thickness of a magnetic layer of a magnetic tape with high resolution by making the magnetic tape, in which the magnetic layer is formed, transmit the light for measuring intensity of the light transmitted.

A monitor display part 5 compares the intensity of laser beam P1, emitted from a light emission part 1, with that of laser beam P2 at a photodetecting part 4, for measuring the coating thickness of a magnetic layer of a sample tape T, and has a calculation processing function built-in. And, when the sample tape T transmits the laser beam P1 and the photodetecting part 4 detects the laser beam P2 transmitted, the monitor display part 5 monitor- displays the intensity of layer beam P2 of the photodetecting part 4, so that, by performing quantitative measurement for the coating film thickness of the sample tape T while a stage 3 is moved in certain direction, line and surface are analyzed while the monitor display part 5 is observed.