PURPOSE: To easily release locking of a hydraulic brake gear in slewing movement or lock it in standstill by a method that the gear is installed between a lower traveling unit and upper slewing unit, and said gear is interlocked with the switching valve of a rotary hydraulic motor.

CONSTITUTION: A hydraulic brake gear 21 is installed between a center frame 6 of a lower traveling unit and a frame 8 of a upper slewing unit which can freely slew on the traveling unit. The gear 21 having a cylinder 22 is supplied with pressure oil from a hydraulic pump 31 through a switching valve 41. Also, the pump 31 supplies pressure oil to a rotary hydraulic motor 30 through a switching valve 33. As both valves 41 and 33 interlock with each other, when the valve 33 is switched to the slewing position, locking of the gear 21 is released, and when the revolution is halted, the gear 21 is locked. In this manner, the slewing and locking can be carried out by a rotary lever alone.