To provide a cylinder block capable of shortening a warming-up period at the time of engine starting.

In a through hole 1s provided in a cylinder block body 1b, a cylindrical metal body 1L constituting a cylinder inner wall is disposed, and an insulating material I is interposed between the metal body 1L and the inner surface of the through hole 1s. Since the heat generated in the cylinder is controlled to transfer by an insulating material, it is possible to shorten the warming-up period at the time of engine starting. Since the metal body 1L is cooled by a cooling medium flowing in a cooling passage P, the deterioration and wear of the metal body by high temperature can be controlled, and the cooling passage P is set to prevent electrolytic corrosion between the metal body 1L and the block body 1b.