PURPOSE: To make it possible to strike in front and strike reversely by means of one and the identical piston.

CONSTITUTION: In a hydraulic down the hole drill's striking mechanism which forms a front oil chamber 3 and a rear oil chamber 4 by installing a piston 1 in a cylinder 2 and connects the front oil chamber 4 to a high voltage circuit 9 and installs a changeover valve 6 behind a piston 1 and changes over the changeover valve 6 by opening and closing a control port 11 by a reciprocating motion of the piston 1 and feeds and discharges high pressure oil to the rear oil chamber 4, thereby driving the hydraulic the piston 1, a reversely striking block 35 is fitted into the rear part of the piston 1 in the cylinder 1. A cylinder liner 30 is slidingly fitted therein in such a fashion that it may be movable in every direction which transfers a retreat starting position and an advance starting position of the piston 1 backward between the outer periphery and the inner periphery of the piston 1 and forces the reverse striking to be carried out.