A spacer covets intermediate portions of respective cylinder bores in a depth direction of a water jacket throughout the entire peripherics of the intermediate portions in the peripheral direction. Accordingly, the intermediate portion of each cylinder bore becomes higher in temperature than any other portion, and is thermally expanded. Thereby, the clearance between the cylinder bore and the corresponding piston increases. Thus, the friction decreases to improve fuel efficiency of an internal combustion engine. Furthermore, the temperature of oil lubricating the intermediate portion of the cylinder bore rises, and the viscosity decreases. Accordingly, the effect of friction reduction is enhanced more. Furthermore, upper and lower portions of the cylinder bores in a cylinder axis direction are sufficiently cooled. Therefore, the cooling performance of a top part and a skirt part of each piston, which tends to become higher in tempera lure, is secured. Accordingly, overheat can be prevented.