A working fluid supply system for a hydraulic power booster and a power steering system includes first and second fluid supply pumps, a hydraulic power boosting apparatus, a power steering unit, various fluid passages connecting thereto and a control apparatus. The first pump supplies pressurized working fluid to a first inlet port of the hydraulic power boosting apparatus. The second pump, controlled by the control apparatus, selectively supplies pressurized working fluid to a second inlet port of the power boosting apparatus to render the fluid supply flow rate applied to the power boosting apparatus substantially constant at a predetermined level when the fluid discharge rate of the first pump is less than the predetermined level. By this, a sufficient amount of pressurized working fluid is supplied to the hydraulic power boosting apparatus to fully operate it under all actual engine speeds. On the other hand, the fluid supplied from the second pump is practically precluded from flowing into the power steering unit due to the arrangement of a control valve means provided in the power boosting apparatus to satisfy the characteristic function of the power steering system.