The invention of the high-pressure chamber in the course of its stroke is converted in accordance with the blood lysate of the rock strength of the stroke of the piston, select one of the two-step stroke of the long-stroke or short stroke and can be automatically switched to, the rock strength in accordance with the to maintain a constant high pressure of the structure is improved so as to improve the stability of the two-stage auto-stroke operation relates to auto-stroke hydraulic breaker. It is arranged so that the cylinder and the top and bottom provided with a low pressure chamber and high pressure chamber is high in the middle provided with a relatively low pressure in the low pressure chamber and a pressure conversion chamber than in the high pressure chamber and; Piston that is coupled to top and bottom can be elevated in the hollow interior of the cylinder is constituted by the small-diameter and large-diameter and; Return formed concavely in the longitudinal direction on the large diameter of the piston and the flow path grooves; From the high-pressure and high-pressure chamber of the lower connecting flow passage connecting the upper portion of the pressure conversion chamber; And a hydraulic circuit for controlling the supply direction of the hydraulic pressure supplied into the cylinder to supply the hydraulic pressure to selectively switch the stroke in accordance with the type of rock; Is consisting of; chisel for the blow to the lower surface of the piston when lowering operation of the piston to the rock crushing.