PURPOSE: To restrain generation of cavitation in a front chamber in a hydraulic impact tool of a front chamber constant pressurizing and rear chamber alternatively pressurizing type.

CONSTITUTION: A piston 3 held in a cylinder 2 free to advance and to retreat forward and backward, front and rear hydraulic chambers 11, 12 formed between a piston outer peripheral part and a cylinder inner peripheral part, a front side hydraulic pressure passage 21 to constantly allow the front chamber 11 to communicate with a hydraulic pump 15 to each other and a rear side hydraulic pressure passage 22 to allow the rear chamber 12 to communicate with the hydraulic pump 15 or the side of a hydraulic tank 16 to each other through a hydraulic pressure change-over valve 14 means are furnished. Negative pressure retraining mechanisms 26, 27 to retrain generation of negative pressure in the front chamber by way of supplying oil to the front chamber at the time when pressure in the front chamber is lowered are provided in a hydraulic breaker devised to repeatedly give impact force to a chisel 5 arranged in front of the piston 3 by way of making the piston repeat advancing and retreating motion in the longitudinal direction by changing over a pressure state of the rear chamber by the hydraulic change-over valve 14.