PURPOSE:To make it possible to judge an abnormality quickly by judging that an abnormality has occurred in the operation of a system when a condition in which a vehicle speed is more than a preset value and a torsional torque is more than a preset value continues longer than a specified judgement time. CONSTITUTION:In a vehicle speed responsive type power steering system, a torsional torque at the time of steering operation is detecting with a torque sensor and power assist is performed in the steering direction according to the detected signal. It is also controlled such that the power assist in the steering direction becomes smaller as the vehicle speed detected with a vehicle speed sensor increases. If in this case a condition in which the vehicle speed is faster than preset value and the torsional torque is larger than a preset value continues more than a specified judgement time, it is determined that an abnormality has occurred in the operation of the power steering system, In the case of such a judgement of abnormality, the steering is changed to manual operation and at the same time a warning is given to a passenger by lighting a failure lamp.