An electric power steering system comprising: a steering force assist device that provides assist force for assisting a steering operation, to a steering system, using a motor as a driving source; and a controller that controls the operation of the steering force assist device, wherein the controller includes a control signal generator ( 18u, 18v, 18w)that generates a motor control signal so as to produce a motor torque corresponding to the assist force, and a driving circuit ( 17u, 17v, 17w) that supplies driving electric power based on the motor control signal, to the motor wherein the control signal generator ( 18u, 18v, 18w) calculates a current command value corresponding to a target assist force, based on a detected steering torque ( T1, T2, T3), and performs a current feedback computation so as to create the motor control signal, wherein the motor is a non-connected brushless motor in which motor coils ( 12u, 12v, 12w) of three phases are not connected to each other, characterized in that the controller includes three systems provided independently of one another with respect to each phase of the motor, each of the systems having the driving circuit ( 17u, 17v, 17w) and the control signal generator ( 18u, 18v, 18w), wherein each of the control signal generators of the three systems converts a current command value of a two-phase coordinate system corresponding to the target assist force, into a phase current command value of each phase, so as to perform phase current feedback control with respect to the corresponding phase, and executes an abnormality determination based on a deviation of a phase current value from the phase current command value, with respect to each of the two phases other than the corresponding phase, and wherein when the control signal generators of two of the three phases determine that an abnormality occurs in the remaining one phase, the controller confirms the determination. a steering force assist device that provides assist force for assisting a steering operation, to a steering system, using a motor as a driving source; and a controller that controls the operation of the steering force assist device, wherein the controller includes a control signal generator ( 18u, 18v, 18w)that generates a motor control signal so as to produce a motor torque corresponding to the assist force, and a driving circuit ( 17u, 17v, 17w) that supplies driving electric power based on the motor control signal, to the motor wherein the control signal generator ( 18u, 18v, 18w) calculates a current command value corresponding to a target assist force, based on a detected steering torque ( T1, T2, T3), and performs a current feedback computation so as to create the motor control signal, wherein the motor is a non-connected brushless motor in which motor coils ( 12u, 12v, 12w) of three phases are not connected to each other, characterized in that the controller includes three systems provided independently of one another with respect to each phase of the motor, each of the systems having the driving circuit ( 17u, 17v, 17w) and the control signal generator ( 18u, 18v, 18w), wherein each of the control signal generators of the three systems converts a current command value of a two-phase coordinate system corresponding to the target assist force, into a phase current command value of each phase, so as to perform phase current feedback control with respect to the corresponding phase, and executes an abnormality determination based on a deviation of a phase current value from the phase current command value, with respect to each of the two phases other than the corresponding phase, and wherein when the control signal generators of two of the three phases determine that an abnormality occurs in the remaining one phase, the controller confirms the determination.