Abstract: A skirt section of the cylinder block is shortened to the height of the rotational center of the crank shaft. To the bottom end of the skirt section is the lower cylinder block whose bottom end is coupled to the oil pan. The inner space of the lower cylinder block section is divided by plural partition walls extending transversely between the side walls of the lower cylinder block section. A bearing cap supporting the crank shaft has a boss section projecting transversely and a threaded hole is formed in the boss section. Further, the bottom end of the bearing cap has a threaded hole having an opening facing bottom. The lower cylinder block section is further provided at its side wall with a hole through which the side portion of the bearing cap is connected to the side wall of the lower cylinder block section. The bottom end of the bearing cap is connected to the side wall of the lower cylinder block section.