Encoder

This invention provides an encoder capable of accurately detecting a failure without adding a position detecting signal. The encoder has a plurality of position detecting signal generating systems (a light source 1, tracks 3a to 3c, and light receiving element units 4a to 4c) for respectively generating electric signals as position detecting signals having different periods, a first electrical angle computing unit 8 and a first position-in-one-rotation data producing unit 9 acting as a first computing unit for computing a first positional data according to the position detecting signals respectively generated by the plurality of position detecting signal generating systems, a second electric angle computing unit 11 and a second position-in-one rotation data producing unit 12 as a second computing unit for computing second positional data according to the electric signals generated by the position detection signal generating systems containing less members of system than the first computing unit, and a compari