Hammering device meant for hammering poles, steel profiles and other elongated objects into the ground which hammering device comprises a body (1) which can be attached to a power tool, a vibrator device (6) which is attached to the body (1) with the help of damping cushions (9) and movable gripping jaws (10) which are adjusted to the mentioned vibrator device (6) in order to grip the object to be hammered at least with a side grip wherein the gripping elements (23) of the gripping jaws (10) can be brought closer and farther away with the help of a regulating unit, such as a hydraulic cylinder (16). In order to maintain the mutual symmetry of the gripping jaws (10) during their movement the gripping jaws (10) essentially comprise arms (2), (3) which are attached to be stiff at the location of the turning joints (8) of the gripping jaws which arms are directed in such a way that the ends of the arms (2), (3) are located next to each other and stay essentially next to each other during the turning movements meant for the gripping jaws (10), and in that the ends of the mentioned arms (2); (3) are connected to each other with the help of a short arm (4) which has a pivot mounting.