PURPOSE: To prevent the increase of a drain pressure even when drain oil is generated owing to internal breakage of one running motor by a method wherein drain oil generated at the running motors on both sides is escaped to a tank through a plurality of drain flow passages.

CONSTITUTION: A rotary joint 41 through which outer and inner valves 43 and 44 are connected to running motors on both sides loaded on a lower running body is provided. Two drain flow passages 45A and 45B are mounted to the inner valve 44 of the rotary joint 41, and drain oil from each running motor is escaped to a tank. This constitution prevents the increase of a drain pressure even when internal breakage occurs to one running motor and quantities of drain oil is generated, prevents breakage of the other running motor, and ensures the strength of the inner valve 44.