A motor control system comprising a motor controller and an encoder, wherein the encoder comprises: a first disk and a second disk which are mounted to a rotation shaft of a motor; and a first rotation position detection unit configured to detect a rotation position in one rotation of the rotation shaft of the motor from the first disk, characterized in that the encoder further comprises: a second rotation position detection unit configured to detect a rotation position in one rotation of the rotation shaft of the motor from the second disk; a rotational speed detection unit configured to detect a rotational speed of the rotation shaft from the second disk; and a transmission unit configured to transmit, according to a predetermined rule, either one of first safety data and second safety data to the motor controller, wherein the first safety data includes first rotation position data indicating the rotation position detected by the first rotation position detection unit and second rotation position data indicating the rotation position detected by the second rotation position detection unit, and the second safety data includes the first rotation position data and rotational speed data including the rotational speed detected by the rotational speed detection unit, and the motor controller comprises: a reception unit configured to receive, from the encoder, one of the first safety data and the second safety data; a rotational speed acquisition unit configured to acquire a rotational speed of the rotation shaft based on the rotation position included in the first rotation position data; and a comparison unit configured to compare the rotation position included in the first rotation position data and the rotation position included in the second rotation position data, when receiving the first safety data, and to compare the rotational speed acquired by the rotational speed acquisition unit and the rotational speed included in the rotational speed data, when receiving the second safety data. a first disk and a second disk which are mounted to a rotation shaft of a motor; and a first rotation position detection unit configured to detect a rotation position in one rotation of the rotation shaft of the motor from the first disk, a second rotation position detection unit configured to detect a rotation position in one rotation of the rotation shaft of the motor from the second disk; a rotational speed detection unit configured to detect a rotational speed of the rotation shaft from the second disk; and a transmission unit configured to transmit, according to a predetermined rule, either one of first safety data and second safety data to the motor controller, wherein the first safety data includes first rotation position data indicating the rotation position detected by the first rotation position detection unit and second rotation position data indicating the rotation position detected by the second rotation position detection unit, and the second safety data includes the first rotation position data and rotational speed data including the rotational speed detected by the rotational speed detection unit, and a reception unit configured to receive, from the encoder, one of the first safety data and the second safety data; a rotational speed acquisition unit configured to acquire a rotational speed of the rotation shaft based on the rotation position included in the first rotation position data; and a comparison unit configured to compare the rotation position included in the first rotation position data and the rotation position included in the second rotation position data, when receiving the first safety data, and to compare the rotational speed acquired by the rotational speed acquisition unit and the rotational speed included in the rotational speed data, when receiving the second safety data.