To relieve an abnormal rise in the internal pressure of a cylinder chamber because of a temperature rise in hydraulic fluid filled into the cylinder chamber, and provide accurate impact force at all times without receiving an effect on the temperature of the hydraulic fluid.

This internal pressure adjusting device for hydraulic impact wrench consists of a pressure relieving chamber 20 which is formed in the cylinder 1 of a hydraulic impact wrench so as to be isolated from a cylinder chamber C and leads a part of hydraulic fluid expanding its volume with temperature rise, and hydraulic fluid passages 16, 17, 19 communicating between the cylinder chamber C and pressure relieving chamber 20, a cam 9 formed on an output shaft 4 fitted into the cylinder 1, and a passage close valve 10 selectively closing the hydrualic fluid passage 16 by bringing it into contact with the cam 9. It is thus possible to block the hydrualic fluid of the cylinder chamber completely so as not to be led into a pressure relieving chamber through the hydrualic fluid passages communicating between the cylinder chamber and pressure relieving chamber if impact force occurs, and prevent the impact force from lowering.