To enable a hole to be efficiently excavated with high vertical accuracy in a short time by controlling a descending speed of a screw rotating drive unit so as not to impose an overload on an auger screw.

The auger screw 8 is inserted into the screw rotating drive unit 5 in a relatively displaceable manner. A screw supporting part 6 is mounted on the drive unit 5 via a load cell 16, and the auger screw 8 is suspended by the supporting part 6. The load cell 16 is connected to a control device 17 provided in a working machine body 2. A support weight detected by the load cell 16 is input into the control device 17, and an actual push-in force exerted on a lower end of the auger screw 8 is calculated by the control device 17. The actual push-in force is compared for computing with a set push-in force input into the control device 17 in advance, and the descending speed of the drive unit 5 is controlled so that the actual push-in force can approach the set push-in force.