PURPOSE: To constitute a hydraulically driven striking mechanism so that the striking times, a returning stroke and coherent striking energy of a striking piston may be automatically applied to the other work conditions by the adjustment of the inside in relation to contact pressure force on the tool insertion end.

CONSTITUTION: A contact pressure control member 31 is temporarily in contact with an insertion end 12 by receiving the action of returning adjusting members 31a, 22 which act in the striking direction, and a control conduit 35 can be connected to a pressure conduit 22 and to a return conduit 25 to be reduced in pressure through the contact pressure control member. A control device 4 performs the control motion in the operating stroke position at an earlier or later point in the time during the striking piston returning stroke, and a switching member 30 can be operated through the control conduit so that the striking piston operating stroke may be released in the striking direction.