An electric power steering system includes: a first rotation detection section including a first sensor section and outputting a signal corresponding to a rotation of an input shaft; a second rotation detection section including second and third sensor sections and outputting signals corresponding to a rotation of an output shaft; and a control unit detecting the respective rotational angles of the input and output shafts and a torque applied to a torsion bar based on the output signals from these rotation detection sections, and providing a driving control of an electric motor. The control unit makes determination on the occurrence of failure at the first to third sensor sections, and provides the driving control of the electric motor as shifting the driving control, based on results of the determination, between a torque control mode using the detected value of the torque and a steering angle control mode using the detected value of the rotational angle of either one of the input and output shafts.