To equalize force holding a sleeve, by uniformizing wall thickness of cast thick parts of a cast cylinder block to eliminate solidifying time difference between cast thick parts in casting, in the cast cylinder block made from light alloy with which a sleeve is cast integrally. The iron cylinder sleeve 2 is cast integrally with an aluminum alloy cylinder block 1, and bolt holes 3a to 3d for tightening the cylinder block 1 are formed in an aluminum casting part covering an outer peripheral part 2a of the sleeve. Connection parts are wide and thick cast connection parts 6a to 6d and connects surrounding parts of the bolt holes 3a to 3d with the outer peripheral part 2a of the sleeve. Thinned parts 8a to 8d are formed in the wide and thick cast connection parts 6a to 6d to uniformize the wall thickness of the aluminum casting part covering the peripheral part 2a of the sleeve.