PURPOSE: To make a percussion mechanism for a drill rod unit usable at an acceptable hole drilling work cost adaptively to various work conditions and, particularly, economically and easily execute such difficult hole drilling work that a strong resistance is applied to a drill rod.

CONSTITUTION: A casing 3 is opened on both sides when viewed from the longitudinal direction of a percussion piston 4 and the piston 4 has a though hole 4b. In addition, drill rods 2, 12, or 13 is passed through the hole 4b. The drill rod has a corresponding surface 9a which receives loads from the piston 4 and the drill rod is divided in longitudinal divisions 12a, 12b, 13a, and 13b and can be operated in such a way that the striking direction of the piston 4 can be inverted by replacing a percussion mechanism 1 against the drill rod.