Abstract: A cylinder block (10) is such that a main body cylinder block (4; 404) and an outer cylinder block (6; 406) are molded separately. As a result, the portion of the die which is used to mold a water jacket (50; 450) does not need to be made thin, thereby increasing the life of the die. Further, cylinders (12; 412) are of a simple shape and the pressure in the axial direction of the cylinder (12; 412) is able to be dispersed substantially evenly between the cylinders (12; 412) and the outer cylinder block (6; 406), thus enabling the cylinders (12; 412) to be thin. Accordingly, there is an increased degree of freedom in design of the portion which forms the water jacket (50; 450). The water jacket (50; 450) and the cylinders (12; 412) can therefore both be made sufficiently thin, thereby contributing to a decrease in both the size and weight of the engine.