A line 93 is provided with a line 71 communicating the line 93 and a hydraulic oil tank 51. The line 71 is provided with a throttle 72 to construct a throttle-provided line 7. When air is trapped between the hydraulic oil tank 51 and the hydraulic pump 52, the hydraulic pump 52 is driven to discharge the air from the throttle-provided line 7 to the hydraulic oil tank 51, thereby removing the air in the hydraulic circuit 5. When the air is removed, resistance caused by the throttle 72 is increased, which makes hydraulic oil flow from a pilot pump 522 to a PPC valve. Since the throttle-provided line 7 realizes air removal with a simple structure, a hydraulic cylinder 44 can be supplied with the air-removed hydraulic oil from an initial stage, thereby enhancing responsiveness of the hydraulic cylinder 44.