A motor control system having a motor control device for controlling rotation of a motor according to a command of a host controller, the system comprising: the host controller for generating a PWM command signal based on a command signal, transmitting the PWM command signal generated and receiving a PWM information signal generated based on an information signal; the motor control device configured to receive the PWM command signal generated based on the command signal, generate the PWM information signal based on the information signal and transmit the PWM information signal generated; wherein the host controller and the motor control device are each containing a PWM modulation section (30, 120) and demodulation section (20, 130); the motor control system further comprising, a signal transmission line (19, 19f, 19s) for transmitting the PWM command signal and the PWM information signal individually as pulse signals; and the motor of which rotation is controlled by the motor control device, wherein the host controller being configured to generate the PWM command signal having a duty factor, that is a ratio of a pulse width to pulse-period duration, modulated by the command signal, and transmit the PWM command signal to the motor control device via the signal transmission line, and the motor control device generates the PWM information signal having a duty factor modulated by the information signal, and transmits the PWM information signal in synchronization with the PWM command signal to the host controller via the signal transmission line, characterized in that the PWM command signal transmitted by the host controller includes a plurality of pulse periods having different pulse-period durations from one another, and wherein the host controller comprises: a PWM period calculating section for calculating a pulse-period duration of the PWM command signal for each of the pulse periods of the PWM command signal; a pulse-width calculating section (34, 124) for calculating a pulse width based on the pulse-period duration calculated by the PWM period calculating section and the command signal; and the PWM modulating section of the host controller is for generating the PWM command signal based on the pulse-period duration calculated by the PWM period calculating section and the pulse width calculated by the pulse-width calculating section, and outputting the PWM command signal. the host controller for generating a PWM command signal based on a command signal, transmitting the PWM command signal generated and receiving a PWM information signal generated based on an information signal; the motor control device configured to receive the PWM command signal generated based on the command signal, generate the PWM information signal based on the information signal and transmit the PWM information signal generated; wherein the host controller and the motor control device are each containing a PWM modulation section (30, 120) and demodulation section (20, 130); the motor control system further comprising, a signal transmission line (19, 19f, 19s) for transmitting the PWM command signal and the PWM information signal individually as pulse signals; and the motor of which rotation is controlled by the motor control device, a PWM period calculating section for calculating a pulse-period duration of the PWM command signal for each of the pulse periods of the PWM command signal; a pulse-width calculating section (34, 124) for calculating a pulse width based on the pulse-period duration calculated by the PWM period calculating section and the command signal; and the PWM modulating section of the host controller is for generating the PWM command signal based on the pulse-period duration calculated by the PWM period calculating section and the pulse width calculated by the pulse-width calculating section, and outputting the PWM command signal.