A motor control system controls a rotation drive of a motor by serially switching a power supply phase of the motor based on a count value of an output signal from an encoder and prevents a temporary noise from causing an abnormal rotation of such a motor. A microcomputer determines that one of an A phase signal or a B phase signal is a noise when the two signals are input at substantially at the same timing and one of the two signals has a shorter time interval from a previous input signal. Thus, a temporary noise is prevented from corrupting normal operation between the encoder count, the rotation position of the motor, and the power supply phase.