A scale includes a scale grating formed with a period P. A light source grating includes a grating formed with a period 2P, the light source grating being disposed between a light source and the scale. Interference fringe detection means is configured to be able to detect a bright part of an interference fringe with the period P, the interference fringe being generated by the light source grating and the scale. The interference fringe detection means detects a first interference fringe formed by light coming from the scale and a second interference fringe formed by light coming from the scale, a position of a bright part of the second interference fringe being shifted from a position of a bright part of the first interference fringe by a half of the period P (i.e., P/2).