A control apparatus for a motor-driven power steering system of a motor vehicle controls the steering system such that a steering torque required to be applied by a driver is prevented from increasing abruptly or steeply even when generation of an assist torque applied to the steering system for assisting the driver in steering operation is to be stopped. The control apparatus includes an operation stop decision mechanism (111, 112, OR) for deciding whether or not an operation stop command is to be issued for stopping control operation for the motor-driven power steering system, a torque detecting mechanism (16, 113) for detecting a steering torque transmitted from an input shaft (21a) to an output shaft (21c) of a steering column (21) of the motor-driven power steering system, and an assist torque control mechanism (114, 116, 117, 118, 13) for updating an assist torque applied by an electric motor (20) to the output shaft (21c) with an assist torque control quantity determined on the basis of a detected torque value outputted from the torque detection mechanism (16, 113), wherein upon issuance of the operation stop command by the operation stop decision mechanism (111, 112, OR), the assist torque control mechanism stops application of the assist torque, provided that the detected torque value is lower than a predetermined value inclusive thereof.