To extend a working range without moving a car body by installing a car body frame projected to one end side in the front and the rear of the car body, a working machine being annexed to the projecting car body frame and having two slewing devices and an outrigger device receiving reaction at the time of the excavation of the working machine.

The hydraulic excavator 1 is composed of a car body section 2, a working machine section 10 mounted on one of the front and the rear of the car body section 2 and a blade section 50 set up to the other. In the operation of the excavtor 1, an outrigger device 8 and a blade 51 are grounded and the excavator 1 is stabilized prior to excavation. Oil pressure is forwarded to a hydraulic cylinder 35 for a support frame, the disk plate 15a of a first slewing device 15 is operated so as to run parallel with a ground surface, and oil pressure is sent to a hydraulic cylinder 36 for a boom and a second slewing device 30 is moved in height, in which the slewing device 30 is easy to be operated. A hydraulic cylinder 37 for an arm and a cylinder 38 for a bucket are supplied with oil pressure, and excavation works are conducted. Accordingly, a working range can be extended without shifting a car body.