To realize an optical position detector capable of being manufactured at a low cost and accurately detecting a position or the like.

The optical position detector 40 arrives a light emitted from a light emitting diode 2 of a light source at a slit group 5a of a fixed slit plate 21 through a diffraction grating formed of a slit group 4a of a moving slit plate 4 and a condenser lens 41, and focuses on a photodetecting surface 3a of a semiconductor position detector 3 through the slit group 5a. A secondary light, a tertiary light or the lie formed of the grating are masked by the fixed slit plate 21. Since a curvature of the lens 41 is set at a curvature r1 of the refractive surface of the fixed slit plate side larger than a curvature r2 of a refractive surface of the moving slit plate side, movement of the plate 4 is amplified, and transmitted to the photodetecting surface 3a. Accordingly, the position can be accurately detected.