flow path of the second hydraulic pump p 2 and shifted to control hydraulic fluid fed to a left traveling device and working devices arm boom option device and so forth a third switching valve c composed of valves installed in a flow path of the third hydraulic pump p 3 and shifted to control hydraulic fluid fed to working devices swing device and so on a confluence switching valve 8 installed on a downstream side of the flow path of the third hydraulic pump p 3 and shifted to selectively supply the hydraulic fluid from the third hydraulic pump p 3 to the working devices on the first hydraulic pump side p 1 or the working devices on the second hydraulic pump side p 2 in response to a pilot signal pressure pi 1 applied through a pilot signal line 31 a first shuttle valve 41 selecting any one of a pressure of a first signal line 34 in which a signal pressure is formed when the third switching valve c for the working devices connected to the third hydraulic pump p 3 is shifted and a pressure of a third signal line 32 in which a signal pressure is formed when switching valves a and b for the working devices connected to the first and second hydraulic pumps p 1 and p 2 are shifted and a second shuttle valve 42 selecting any one of the pressure selected by the first shuttle valve 41 and a pressure of a second signal line 33 in which a signal pressure is formed when a switching valve d for the traveling devices is shifted.