To provide a cylinder block and its processing method having little adverse effect on the natural environment and capable of retaining a lubricating oil sufficiently on an inner surface of a cylinder bore.

The processing method for the cylinder block 10 of an internal combustion engine is to process the inner surface 11a of the cylinder bore 11 formed on the cylinder block 10, and comprises a honing process (step ST1) to perform honing of the inner surface 11a of the cylinder bore 11 while a water-soluble coolant is fed, and an oil film forming process (step ST2) to form an oil film 11b on the inner surface 11a of the cylinder bore 11 after being subjected to the honing.