To prevent a slewing base from moving immediately after finishing the front attachment operation.

A brake-release cylinder 19 for a hydraulic motor 3 is controlled by an open/close valve 37 having a pilot chamber 39 in which pilot pressure is changed in response to the operation of direction changeover valves 14, 12a, 12b, 13a and 13b for the front attachment and a pilot chamber 38 in which pilot pressure is changed by the operation of a slewing operating device 18. In the operation of the front attachment and slewing, a brake of the motor 3 is immediately released by the cylinder 19. Upon the completion of the slewing operation, the motor 3 is braked after stopping the operation of a relief valve 42b or 42a. Immediately after finishing the front attachment operation, a brake is applied to the motor 3.