PURPOSE: To continuously reduce the output of a hammering mechanism, and simultaneously, to the slightly reduce the hammering times of the hammering mechanism when pressure exceeds a specified limit value based on the load of a rotational mechanism, by improving a hydraulically driven rotary hammer drill.

CONSTITUTION: A first control member 25 for reducing operating pressure for driving a hammering piston 3 when pressure exceeds a limit value of operating pressure of a rotational mechanism, and a second control member 42 for changing extrusion force which acts against that the hammering piston 3 is returned in the direction opposite to the hammering direction (shown by an arrow 20) are provided on a pressure conduit 24 for supplying a driving medium to the hammering piston 3.