A power steering apparatus according to the present invention comprises: a steering mechanism having a steering shaft and a rack bar which steers steer steerable wheels by a movement in an axial direction of the rack bar in association with a rotation of steering shaft; an electrically driven motor having a stator, a rotor, and a drive shaft integrally rotated with the rotor; a worm gear transmitting the rotation of the drive shaft to the steering mechanism; a housing member having a reduction gear housing section, a rack bar housing section, and a motor ECU housing section in which the electrically driven motor and the control circuit are housed; and a moisture detection sensor having a moisture detection section disposed within the reduction gear housing section and a transmission section connected with the control circuit passing through an inner part of the motor ECU housing from the moisture detection section.