To obtain a cooling water passage structure with excellent workability at low cost, making an engine compact, by forming a joint housing with a transmission case at one end of the longitudinal direction of a cylinder block and preliminarily disposing a cooling channel within space surrounded by this joint housing.

A cylinder block 1 of V-type diesel engine is provided with left and right bungs 16a, 16b and a joint housing 2 with a transmission case is formed at the rear end of the longitudinal direction. In space 5 surrounded by this joint housing 2, a cooling channel 6 is disposed by utilizing the inside of space 5 which is the front part of a flywheel and has been dead space. Namely, a cooling channel 6 is partitioned by a cooling water pipe 7 which is formed integrally with a cylinder block 1 by casting and one end 8 of the cooling water pipe 7 is passed through a side surface 9 of the joint housing 2 and is projected to the outside. In an opening part 10 at one end 8, a piping linked with a thermostat is formed so as to be connected.