An electric power steering system comprising: an electric motor that applies a steering assist force for reducing a driver's steering load to a steering system; a torque detecting unit that detects a steering torque; a steering assist command value computing unit that computes a steering assist command value based upon at least the steering torque detected by the torque detecting unit; a clip unit that limits the steering assist command value by use of a predetermined limit value based upon the steering torque detected by the torque detecting unit so that the steering assist command value computed by the steering assist command value computing unit does not exceed an allowable range; a motor control unit that controls driving of the electric motor based upon the steering assist command value limited by the clip unit; characterized by an interlock unit that has a monitoring function of permitting or prohibiting the driving of the electric motor controlled by the motor control unit based upon the steering torque detected by the torque detecting unit; a torque abnormality detecting unit that detects abnormality of the steering torque detected by the torque detecting unit; an alternative torque value computing unit that, when the abnormality is detected by the torque abnormality detecting unit, computes an alternative steering torque value based upon the steering torque that is normal, which is detected by the torque detecting unit, when the abnormality is not detected; and an input switching unit that switches an input value to the clip unit and an input value to the interlock unit from the steering torque detected by the torque detecting unit to the alternative steering torque value computed by the alternative torque value computing unit, when the abnormality is detected by the torque abnormality detecting unit. an electric motor that applies a steering assist force for reducing a driver's steering load to a steering system; a torque detecting unit that detects a steering torque; a steering assist command value computing unit that computes a steering assist command value based upon at least the steering torque detected by the torque detecting unit; a clip unit that limits the steering assist command value by use of a predetermined limit value based upon the steering torque detected by the torque detecting unit so that the steering assist command value computed by the steering assist command value computing unit does not exceed an allowable range; a motor control unit that controls driving of the electric motor based upon the steering assist command value limited by the clip unit; characterized by an interlock unit that has a monitoring function of permitting or prohibiting the driving of the electric motor controlled by the motor control unit based upon the steering torque detected by the torque detecting unit; a torque abnormality detecting unit that detects abnormality of the steering torque detected by the torque detecting unit; an alternative torque value computing unit that, when the abnormality is detected by the torque abnormality detecting unit, computes an alternative steering torque value based upon the steering torque that is normal, which is detected by the torque detecting unit, when the abnormality is not detected; and an input switching unit that switches an input value to the clip unit and an input value to the interlock unit from the steering torque detected by the torque detecting unit to the alternative steering torque value computed by the alternative torque value computing unit, when the abnormality is detected by the torque abnormality detecting unit.