As a thickness measurement using the resonance of microwaves, a method using a cylindrical type resonator has been reported. It offers the measurement with high accuracy. But since this method requires a very close separation between the object and the resonator, it seems inconvenient in the case of high temperature objects. Therefore the present authors fix their eyes on Fabry-Perot resonator as a method with little limitation of the distance from the resonator to the object, and have developed a thickness meter which consists of two displacement meters using a Fabry-Perot resonator. The manufactured thickness meter offered good performance as follows: (1) Accuracy is 20 µm, (2) resolution is 2 µm, and (3) maximum distance between a plate and thickness meter is 107 mm. The meter was applied to the transformation measurement of steels, and it is found that the meter can be used for measuring the transformation. The behavior of the transformation was clearly detected.