An encoder includes a first rotator connectable to a rotating shaft and including a first pattern, and a first detection unit positioned at a non-rotatory member and configured to detect the first pattern. The encoder also includes a second rotator connectable to an output shaft that is rotatable around a predetermined axis and is configured to output rotation caused by the rotating shaft. The encoder also includes a second detection unit positioned at the non-rotatory member and configured to detect the second pattern, and a reference unit configured to position the first detection unit and the second detection unit by a common position reference.