An encoder includes a first multiple rotation counter that generates first multiple rotation data using a rotation signal indicating one rotation of a rotational shaft of a motor, a second multiple rotation counter that generates second multiple rotation data using the rotation signal, a first cumulative-number calculation unit that calculates first accumulated multiple rotation data using an angle signal indicating a rotational angle of the rotational shaft, a second cumulative-number calculation unit that calculates second accumulated multiple rotation data using the angle signal, and a first comparative diagnosis unit that diagnoses whether the encoder has a fault by performing a comparison to determine whether the first multiple rotation data, the second multiple rotation data, the first accumulated multiple rotation data, and the second accumulated multiple rotation data are a same value.