To provide a split type cylinder block capable of reducing a pumping loss through the medium of a breathing hole of the partition wall and improving a strength reliability of the partition wall.

The cylinder block 1 is equipped with a crankcase 31, an exterior wall 32, partition walls 38A-38C, breathing holes H1, H2, a block body 3 integrally formed including cylinder mounting portions PA-PD, and a cylinder structure 5 formed independent from the block body 3. The cylinder structure 5 is formed in such a manner that a supporting bottom surface 51UA of cylinder bottom surfaces which is opposed to tops of the side walls 37A, 37B and the partition walls 38A-38C is protruded from the other bottom surfaces in the axial direction of a cylinder through the medium of the cylinder mounting portions PA-PD. In such a state that the cylinder structure 5 is assembled to the block body 3, the supporting bottom surface 51UA is abutted to the cylinder mounting portions PA-PD disposed at the tops of the side walls 37A, 37B and partition walls 38A-38C.