A pressure medium-operated percussion device of a rock breaking device arranged to provide impact pulses to a rock breaking tool, the percussion device comprising: a frame; a percussion piston that is during its work cycle arranged to move axially back and forth in the impact direction and return direction, when the pressure medium acts on its working pressure surfaces (24, 25); at least one pressure-controlled sleeve-like control valve that is arranged around the percussion piston and has positions for directing the pressure medium to act on at least one working pressure surface of the percussion piston and away from it for the purpose of controlling the work cycle of the percussion piston, the control valve being equipped with pressure surfaces (36, 37) and the control valve being arranged to move between its extreme positions under the influence of the pressure medium acting on the surfaces; at least one impulse space (34, 35) that comprises an annular space between the percussion piston and control valve; at least one surface (K1, K2) on the percussion piston which defines the impulse space (34, 35) in the axial direction and reduces the volume of the impulse space as the percussion piston moves toward it; the impulse space (34, 35) being in a pressure connection with at least one pressure surface (36, 37) of the control valve located in the impulse space (34, 35); and the impulse space (34, 35) having at least one pressure connection (Y1, Y2) for the flow of the pressure medium from the impulse space; at least one impulse edge (S1, S2) on the percussion piston for closing the pressure connection to the impulse space (34, 35), whereby the impulse space becomes a closed pressure space and a pressure pulse is generated; the pressure pulse affecting the control valve and providing its alternating movement an initial impulse toward its opposite extreme position; wherein the percussion device comprises a first impulse space in the impact direction, and the percussion piston has impulse edges (S1, S2) in both travel directions (A, B); characterized in that the percussion device further comprises a second impulse space in the return direction; whereby the control valve is provided an initial impulse of the alternating movement in both its extreme positions. a frame; a percussion piston that is during its work cycle arranged to move axially back and forth in the impact direction and return direction, when the pressure medium acts on its working pressure surfaces (24, 25); at least one pressure-controlled sleeve-like control valve that is arranged around the percussion piston and has positions for directing the pressure medium to act on at least one working pressure surface of the percussion piston and away from it for the purpose of controlling the work cycle of the percussion piston, the control valve being equipped with pressure surfaces (36, 37) and the control valve being arranged to move between its extreme positions under the influence of the pressure medium acting on the surfaces; at least one impulse space (34, 35) that comprises an annular space between the percussion piston and control valve; at least one surface (K1, K2) on the percussion piston which defines the impulse space (34, 35) in the axial direction and reduces the volume of the impulse space as the percussion piston moves toward it; the impulse space (34, 35) being in a pressure connection with at least one pressure surface (36, 37) of the control valve located in the impulse space (34, 35); and the impulse space (34, 35) having at least one pressure connection (Y1, Y2) for the flow of the pressure medium from the impulse space; at least one impulse edge (S1, S2) on the percussion piston for closing the pressure connection to the impulse space (34, 35), whereby the impulse space becomes a closed pressure space and a pressure pulse is generated; the pressure pulse affecting the control valve and providing its alternating movement an initial impulse toward its opposite extreme position; wherein the percussion device comprises a first impulse space in the impact direction, and the percussion piston has impulse edges (S1, S2) in both travel directions (A, B); characterized in that the percussion device further comprises a second impulse space in the return direction; whereby the control valve is provided an initial impulse of the alternating movement in both its extreme positions.