A cooling water passage structure of internal combustion engines is provided which can contribute to a scale-down of the external form of the engine and also can prevent deformation of the cylinder bores. The cooling water passage structure comprises a water jacket for the flow of cooling water provided on the peripheries of a plurality of cylinder bores; and a lateral suction passage 9a, 9b integrally formed with a side wall of a cylinder block 2 having bolt holes formed for fastening a cylinder head 3. The lateral suction passage runs through by the cylinder bores at a height in the vicinity of the water jacket bottom and is arranged on the outside of, and perpendicular to the bolt holes. On the rear surface of the cylinder head 3 is mounted a water passage block 15 and on the front end part of the cylinder block 2 is provided a water pump housing 10.