To effectively and precisely inspect the surface deformation of a belt.

The optical deformation measurement device 16 measures the surface deformation of the belt 10 which is applied to the pulleys 12 and 14. The data measured by the deformation measurement device 16 is input to the deformation calculation part 24 for calculating the surface deformation of the belt 10. In the memory part 22 of the deformation detection device 20, the information of position and deformation is previously stored. When the measurement data is input, the deformation calculation part 24 calculates the amount of surface deformation of the belt 10 from the measured data and the information of position and deformation of the surface of the pulley read from the memory part 22.