PURPOSE: To decrease the oil temperature of an oil pressure pump, so as to prevent seizure, by respectively communicating an inflow port and an outflow port of a relief valve with the inflow port side and the outflow port side of an oil pressure motor, and communicating the upstream side of the oil pressure motor with a tank when the circuit pressure becomes higher than the set pressure of the relief valve.

CONSTITUTION: A relief valve R is integrally incorporated into an oil pressure motor M, and an inflow port 7 is communicated with an inflow port 4 side of the oil pressure motor M, and an inflow port 8 is communicated with an outflow port 6 side of the oil pressure motor M. Accordingly, operating oil on the upstream side of the oil pressure motor M is returned into a tank T through the suction port 7 and the outflow port 8 of the relief valve R when the circuit pressure becomes higher than the set pressure of the relief valve R. The operating oil returned into the tank T is sucked to an oil pressure pump P after being cooled in the tank T. Since cooled operating oil is always supplied into the oil pressure pump P, the temperature rise of the oil pressure pump P is avoided, and seizure is not generated.