In an electric power steering system (10) for use in a vehicle incorporated with a vehicle behavior control system (30), a failure detecting unit (12) detects a failure of the electric power steering system according to the steering torque and actual electric current. A criterion for detecting a failure by the failure detecting unit is changed when the vehicle behavior control system is activated. Thereby, even when the vehicle behavior control system is activated and it causes a corrective current to be supplied to the electric motor, because the failure detection criterion is changed, the failure detecting unit is prevented from confusing the corrective current due to the operation of the vehicle behavior control system with an erroneous current caused by a failure of the electric power steering system. Therefore, an unnecessary interruption of the operation of the electric power steering system can be avoided, and the convenience of the electric power steering system can be enhanced.