A portable machine tool, especially a hammer drill, mainly comprises a housing in which a guide tube is arranged. A piston and a beater are guided in the guide tube reciprocatable in axial direction, the piston and the beater having facing axially spaced ends forming therebetween in the guide tube an air pillow through which the impact energy of the piston is transmitted to the beater. A turning sleeve coaxially surrounds the guide tube and is provided at its front end with a holding bushing formed with a coaxial bore therethrough in which a shaft of a tool, for instance a drill, is mounted for rotation therewith and for limited axial movement relative thereto. The beater is provided at the front end with a beater bolt having an outer diameter slightly smaller than the inner diameter of the bore so that the beater bolt, which transmits its impact energy directly to the free end of said tool shaft, may enter into the bore to be guided therein during reciprocation of the beater.