PURPOSE: To reverse braking pressure to driving pressure for eliminating drastic hydraulic variation by short-circuiting the high pressure side circuits of left and right hydraulic drive units through an equalizing valve in steering and delaying equalizing function of the valve in returning the steering.

CONSTITUTION: Left and right hydraulic drive units 1, 2 are provided respectively with variable pumps 3, 8, variable motors 4, 9 and shuttle valves 7, 12. Outlet ports 7a, 12a of both shuttle valves 7, 12 are connected to each other through pipe paths 14, 15 and an equalizing valve 13. And for example in right steering, a solenoid of the equalizing valve 13 is turned off to interconnect both pipe paths 14, 15. Thus, the high pressure side circuits of the left and right hydraulic drive units 1, 2 are short-circuited. When the steering is returned, equalizing function is delayed by the operation of a timer 18 so that time in which braking pressure of the left hydraulic drive unit 1 is reversed to driving pressure will be given. Thus, the braking pressure is reversed to the driving pressure to eliminate drastic hydraulic variation.