When a convex lens 13 comes close to 0-th order transmitted light T0 with rotation of an optical scale 11, the 0-th order transmitted light T0 derived from a light beam L emitted from a light source 15 and transmitted through a diffraction grating 12 around is converged and incident on a reference position signal detection sensor 14. At that time, a large output is obtained from the reference position signal detection sensor 14. This signal constitutes a reference position signal. On the other hand, reflective diffracted light beams La, Lb having been reflected by the diffraction grating are detected by a diffraction light detection sensor 16 after interference, so that the speed and the angle etc. of the scale 11 are detected. In this way, the reference position signal and an incremental signal are detected at the same position on the optical scale.