A power steering system for a vehicle. The system includes a hydraulic
power cylinder, first and second hydraulic lines, a bypass passage
through which the first and second hydraulic lines are connected with
each other, and a flow passage change-over device adapted to close the
bypass passage during steering assisting. A supply passage and a return
passage are disposed in parallel with each other to form part of each of
the first and second hydraulic lines. The flow passage change-over device
is disposed in each of the first and second hydraulic lines. A
differential pressure regulating valve is disposed in each of the return
passages and adapted to allow hydraulic fluid to flow toward the
hydraulic pressure source in a condition where pressure on side of the
hydraulic power cylinder becomes higher than a set pressure or higher
than pressure on side of the hydraulic pressure source.