An encoder, which measures a relative moving amount of a second member relative to a first member, includes: a diffraction grating provided on the first member; a light-incident optical member causing a measuring light to come into a grating pattern surface of the diffraction grating substantially perpendicularly; a first reflecting member provided on the second member and reflecting a diffracted light generated from the diffraction grating; a first direction-changing member changing a direction of the diffracted light; a first photo-detector detecting an interference light generated by interference between a double diffracted light and other diffracted light than the double diffracted light or a reference light, the double diffracted light being generated, via diffraction of the diffracted light, from the diffraction grating; and a measuring section which obtains the relative moving amount of the second member relative to the first member by using a detection signal from the first photo-detector.