PURPOSE: To raise the efficiency of soil discharge operation by a method in which when soil and sand are discharged from a collecting chamber, a throttle actuator to lower the rotation number of an engine for driving a blower is controlled in an interlocking manner with the opening operation of the door of the soil discharge port.

CONSTITUTION: During the discharge operation by suction of soil and sand, a piston cylinder mechanism 20 is driven by a signal from a controller 30, a door 17 is horizontally operated through a piston rod 19, and a soil discharge port 16 is opened. Deposited soil in the collecting chamber 5 is dropped and discharged, and the door 17 is closed. In this case, when the door 17 is opened, a signal is given from a controller 30 to a throttle actuator 31, the rotation number of the engine 2 is lowered by operation of the throttle, and the suction force of the blower 3 is temporarily dropped. The continuity of soil discharge operation by suction can be retained without sucking up soil and sand from the soil discharge port 16.