PURPOSE: To allow movement while an outrigger is pulled out and grounded, to increase the efficiency of pile driving work and to prevent the falling of a truck by providing a pile driving mechanism in a crane main body and a plurality of rollers in the base seat of the outrigger of an on-truck crane.

CONSTITUTION: Two rollers 7 are fixed to the grounding surface of the base seat 6 of the outrigger 5 of a crane 2 loaded on a truck 1. When the truck 1 is moved to a next driving position after a pile 10 is driven by a pile driver 4 provided in the main body of the crane 2, the truck 1 is moved by keeping the grounded state of the base seat 6 of the outrigger 5. Thus, the pile 10 is efficiently driven by the pile driver 4 without drawing/housing the outrigger 5, and the falling of the truck 1 caused by the weight of the pile driver or the elongated boom of the crane 2 is prevented by the rollers 7 of the outrigger 5 during movement.