A power steering apparatus includes: a torque sensor for detecting a steering torque applied to a steering wheel and outputting a torque signal corresponding to the steering torque; a speed sensor for detecting a speed of a vehicle and outputting a speed signal corresponding to the speed of the vehicle; a humidity sensor for outputting a humidity signal corresponding to a humidity of an interior of the power steering apparatus; and an electronic control unit for receiving the humidity signal from the humidity sensor, and for, when a measured humidity is an reference value or higher, generating a motor current signal for supplying an assistant steering force smaller than an assistant force set in response to the torque signal and the speed signal.