A malfunction determination device for a motor control system, outputs a position signal that indicates a moved position of a movable member of a motor and a direction signal that indicates a movement direction of the movable member. The malfunction determination device determines a malfunction has occurred if a relationship between the position signal and the direction signal differs from that when the motor control system is operating normally. The malfunction determination device outputs a pulse signal that varies periodically along with variations in the moved position of the movable member as the position signal; outputs a periodic pulse signal in the same phase with the position signal as the direction signal if the movable member is moving in one direction; and outputs a periodic pulse signal in an opposite phase to the position signal as the direction signal if the movable member is moving in the other direction.