The invention relates to an impulse generator for a rock breaking tool, the impulse generator (2) comprising a main chamber (4) for receiving a first pressurizeable fluid volume (6), an in the main chamber (4) received impulse piston (8) which is arranged for transfer of pressure energy in the fluid volume (6) into impulses in the tool (10), and a on the side opposite the main chamber (4) side of the impulse piston (8) situated prepressurizing chamber (12) for receiving a second pressurizeable fluid volume (14), where the impulse generator (2) further comprises a on the side opposite the main chamber (4) side of the impulse piston (8) situated pressure relief chamber (16) for receiving a third pressurizeable fluid volume (18), where the relationship between the pressurizing pressures in the fluid volumes (6,14,18) and the relations between the areas of the impulse piston (8) facing the chambers (4,12,16) are such that pressurizing of at least the prepressurizing chamber (12) displaces the impulse piston (8) in the direction towards the main chamber (4) and the pressure in the main chamber (4) effects a pressure increase in the pressure relief chamber (16) when the prepressurizing chamber (12) is depressurized, whereby the depressurizing rate in the pressure relief chamber (16) and the velocity of the then transferred pressure impulse into the tool (10) are increased (12). The invention also relates to a hydraulic tool comprising an impulse generator (2), and a method for generation of impulses in a rock breaking tool.