An encoder (10) includes a sensor (1) configured to illuminate light on a single scale (100) to detect two phase signals having periods different from each other, an error signal generator (2) configured to arrange accumulated amounts of phase changes of the two phase signals when the scale (100) and the sensor (1) are relatively displaced from each other by a predetermined displacement so as to obtain two accumulated phase signals and to obtain a difference between the two accumulated phase signals so as to generate an error signal that is obtained by removing an accumulated phase component corresponding to the predetermined displacement, and an interpolation error detector (3) configured to average the error signal by using at least one of periods of the two phase signals so as to detect an interpolation error contained in at least one of the two phase signals.