To reduce price and vibration of a hammer and cope with the deterioration caused by heat generation of a piston during operation related to heat transmission of a piston material.

This rotary hammer is provided with an air cushion hammer mechanism provided with a tool holder, a piston 9, and a beat piece hitting a tool in the tool holder due to reciprocating movement of the piston in a cylinder 8' and a means for rotating the cylinder for rotating a tool. The piston 9 is formed of a plastic material, an annular seal 27 provided in an annular groove 26 of the piston 9 has a larger inside diameter than a diameter of a face facing the outside in radial direction of the groove 26 to seal the piston 9, and a dimension of the groove 26 in the axial direction is larger than the dimension of the annular seal 27 so that the seal rotates together with the cylinder 8' when the cylinder rotates around the piston 9 (without reciprocating movement of the piston).