PURPOSE: To absorb casting distortion by machining slits on cylinder barrels prior to the true circle machining when true-circle-machining the inner peripheries of cylinder sleeves after casting cylinder sleeves in multiple cylinder barrels respectively.

CONSTITUTION: A cylinder block 1 has a cylinder block main body 2 made of aluminum alloy and connected with four cylinder barrels 3-6 in series and cylindrical cylinder sleeves 7 made of cast iron and cast in the cylinder barrels 3-6 and forming cylinder bores 8 on the inner peripheries. Circular arc-shaped slits 16 connecting adjacent multiple communicating ports 14 are formed on the upper deck 12 of the cylinder block main body 2. In this case, the true circle machining for the cylinder bores 8 is performed after the slits 16 are formed. The distortion generated when the cylinder block 1 is cast is absorbed by the slits 16.