A water circulation structure of a cylinder block having a compact layout of piping connecting an engine body and a radiator. A water-cooled internal combustion engine is provided with an engine body including a cylinder block and a cylinder head and a radiator. The radiator is disposed to be separated, in a prescribed direction, i.e. in a rightward direction, from the engine body. A cooling water outlet portion is open to a cylinder head water jacket and is provided in the end portion of the cylinder head. The cooling water outlet portion being connected with an inlet pipe for leading the cooling water flowing out of a cylinder block water jacket into the cylinder head water jacket to the radiator. The cooling water outlet portion is disposed to the right closer to the radiator than a chain chamber.