PURPOSE: To enable to make a straight forward running even under work by a method wherein the pilot pressure receiving part of a selector valve between right and left traveling motors and the pilot pressure receiving part of the remaining valves are communicated to each other through selector valves.

CONSTITUTION: A left and a right traveling valves 4L and 4R, which control the supply of pressure oil to the left and the right traveling motors 12 and 14, and the remaining valves, which control the supply of pressure oil to cylinders for working machinery 15W17 and to a slewing motor 11, are connected each other in parallel. Each of said remaining valves has a construction to be changed-over with pilor pressure. Furthermore, the selector valve 20, which controls the forward ports 12a and 14a and the backward ports 12b and 14b of the left and the right traveling motors 12 and 14 by making and breaking, is provided. The selector valve 20 is of normally shut-off position and shifted to communicated position by means of a pilot pressure. The pilot pressure receiving part 22 and the pilot pressure receiving parts of the remaining valves are communicated to each other through the selector valves 30 and 31. At the same time, the selector valves 30 and 31 take communicated positions only when the left and the right traveling valves 4L and 4R are operated to change-over to the same position.