A joint of a robot includes a driving motor, a speed reducer that reduces rotation of an output shaft of the motor, and an arm connected to the speed reducer. The joint includes an input encoder that detects a rotational position of the motor and an output encoder that detects a rotational position of the arm. A controller controls the motor based on data indicating the rotational positions of the motor and the arm detected by the input encoder and the output encoder, respectively. The input encoder is arranged with a register that latches and retains an input EC value. The output encoder includes a register that latches and retains an output EC value, and an edge pulse output section that outputs an edge pulse to both registers every time the output EC value changes. In response to the edge pulse, both registers concurrently output EC values to the controller.