Disclosed is a power steering device having at least two redundant steering torque sensor units, at least two redundant steering angel sensor units, at least two redundant motor position sensor units and a control unit configured to: in a normal state, perform steering assist control based on a steering torque detection signal of one of the steering torque sensor units, a steering angle detection signal of one of the steering angle sensor units and a motor position detection signal of one of the motor position sensor units; perform redundant monitoring on the steering torque sensor units, the steering angel sensor units and the motor position sensor units; and, upon detection of an abnormality in any sensor output signal by the redundant monitoring, switch the abnormal signal to an alternative signal.