A hammer drill (100) comprises an electric motor (110), a piston (127) as a driving member and a controller (199) which controls and drives the electric motor (110). The piston (127) is reciprocated by rotation of the electric motor (110) and thereby a hammer bit (119) is driven in its longitudinal direction. The hammer bit (119) is moved forward in response to a forward movement stroke of the piston (127) and thereby a hammering operation is performed by the hammer bit (119). The controller (199) sets a duty ratio of a driving pulse signal which drives the electric motor (110) in the forward movement stroke of the piston (127) to be larger in order to prevent a reduction of a rotation speed of the electric motor (110) due to a load applied on the electric motor (110) during the forward movement stroke of the piston (127).