PURPOSE: To provide a linerless cylinder block structure which is easy to be cut while keeping abrasion resistance of a piston sliding face, by forming a preform only on a part adjacent to a dead point on the piston without forming any preform all over the piston sliding face.

CONSTITUTION: In a linerless cylinder block 1 in which a preform 2 is formed on the sliding face of a piston 3 in the inner wall of a cylinder bore so as to improve abrasion resistance, the preform 2 is so constituted as to be formed only on the piston sliding face near the upper dead point of the piston.