PURPOSE: To improve the measuring accuracy of the thickness of a film by making the amount of reflected light constant even if an object to be measured is slightly inclined.

CONSTITUTION: This device is constituted of a stage 5, which mounts an object to be measured 1 and has the mounting surface whose inclination can be changed, a light source, which emits incident light 2 on the object to be measured 1, a photodetector 4, which detects reflected light 3 from the object to be measured 1, and a feedback circuit 6, which keeps the amount of the reflected light constant by feeding back the fluctuating part of the amount of the reflected light caused by the movement of the stage 5 and changing the inclination of the stage 5.