A power steering system includes first and second communication passages for providing communication between a first oil passage and a second oil passage, a third communication passage for providing communication between the first and second communication passages, a directional control valve disposed in the third communication passage, a first check valve disposed in the first communication passage for allowing flow from the first oil passage to a first connection portion only, a second check valve disposed in the first communication passage for allowing flow from second oil passage to the first connection portion only, a third check valve disposed in the second communication passage for allowing flow from a second connection portion to the first oil passage only, and a fourth check valve disposed in the second communication passage for allowing flow from the second connection portion to the second oil passage only.