Power steering system including: an electric pump to supply fluid to a power steering unit; and an engine control unit (ECU) to control an amount of the fluid by a rotation speed of the electric pump. The ECU includes a first device for calculating an on-slope standby rotation speed and setting the electric pump to this speed when a vehicle is descending or ascending a slope. This speed is calculated by correcting a standby rotation speed of the electric pump corresponding to a vehicle speed with a slope correction value calculated according to a slope angle or a vehicle body angle. Accordingly, it is possible to supply the fluid with the electric pump and assist a steering operation to suppress both a jerky steering feeling due to insufficient steering force in slope descent and a lack of response in the steering operation due to excessive steering force in slope ascent.