a rock splitter of oil hydraulic piston type includes a housing having a plurality of cylinder chambers on the upper portion and first and second paths communicated with the cylinder chambers for supplying and discharging oil ; a piston in the cylinder chamber of the housing ; and a cap capable of a vertical movement , the cap being connected to the upper surface of the housing in such a manner that the inner surface thereof is in contact with the upper surface of the housing . the cap moves vertically depending on the movement of the piston . the rock splitter further comprises : a cylinder detachably mounted inside the cylinder chamber of the housing ; a collapsible member having a locking member detachably mounted in the cylinder chamber , a plurality of extension members capable of a vertical extension ; and an elastic member supporting the lower end of a central extension member . the entire front surface area of the cap touches the rock to maximize the power applied to the rock . repair to the cylinder is effected by replacing only the cylinder .