PURPOSE: To permit the actuation of a turning motor during the flat land running period by connecting in parallel a running direction switch valve with a turning direction switch valve while throttling a supply circuit to the running direction switch valve when the discharge pressure of the turning direction switch valve is high.

CONSTITUTION: A turning direction switch valve 16 and a left side running direction switch valve 19 are connected to an oil-pressure pump, and the center bypass port C7 of the running direction switch valve 19 is connected to a center bypass 42 downstream of the turning direction switch valve 19. When the pressure of thee discharge circuit of the turning direction switch valve 16 is high, the turning and running switch valves 16 and 19 are connected in parallel while throttling a supply circuit 43 to the running direction switch valve 19. Also, when the discharge pressure is low, a switch valve 39 to connect in series the turning direction switch valve 16 with the running direction switch valve 19 is provided in parallel with the discharge pipeline 20 of an oil-pressure pump and on the upstream side of the turning direction switch valve 16.