To eliminate any necessity of changing a measuring method of a displacement sensor or replacing the displacement sensor according to the state of reflection of a surface of an object to be measured.

A diffuse reflection plate 212 is disposed at a position on which probe light that is emitted from a laser light source 231 and fully reflected on a working face of a work-piece 102, is incident. A linear sensor 233 receives first reflected light that is formed by further reflecting the probe light that has been diffused and reflected by the diffuse reflection plate 212, on the working face of the work-piece 102, and second reflected light that is formed by diffusing and reflecting the probe light that has been emitted from the laser light source 231, on the working face of the work-piece 102. A signal processing unit 234 detects a first distance of the probe light from being exited by the first reflected light till entering the diffuse reflection plate 212 via the working face of the work-piece 102, or detects a second distance of the probe light from being exited by the second reflected light till entering the working face of the work-piece 102.