An internal combustion engine comprising: an oil pan: a crankcase, the crankcase being formed with an opening that communicates between the crankcase and the oil pan and being provided with an oil pan side fastening area that surrounds the opening and contacts the oil pan; and a balance shaft housing that is disposed between the oil pan and the crankcase to cover the opening, wherein the crankcase is provided with plural housing side fastening sections that fasten the crankcase and the balance shaft housing, the fastening section of the plural housing side fastening sections in a first axial direction side of the balance shaft is disposed on an inside of the oil pan side fastening area, and the fastening section of the plural housing side fastening sections in a second axial direction side of the balance shaft, that is opposite from the first axial direction, is disposed on an outside of the oil pan side fastening area; characterized in that a speed changing mechanism is disposed in the second axial direction side in the crankcase, the fastening section of the plural housing side fastening sections on an opposite side from the second axial direction side where the speed changing mechanism is disposed is disposed on the inside of the oil pan side fastening area, and the fastening section of the plural housing side fastening sections in the second axial direction side where the speed changing mechanism is disposed is disposed on the outside of the oil pan side fastening area. an oil pan: a crankcase, the crankcase being formed with an opening that communicates between the crankcase and the oil pan and being provided with an oil pan side fastening area that surrounds the opening and contacts the oil pan; and a balance shaft housing that is disposed between the oil pan and the crankcase to cover the opening, wherein the crankcase is provided with plural housing side fastening sections that fasten the crankcase and the balance shaft housing, the fastening section of the plural housing side fastening sections in a first axial direction side of the balance shaft is disposed on an inside of the oil pan side fastening area, and the fastening section of the plural housing side fastening sections in a second axial direction side of the balance shaft, that is opposite from the first axial direction, is disposed on an outside of the oil pan side fastening area; a crankcase, the crankcase being formed with an opening that communicates between the crankcase and the oil pan and being provided with an oil pan side fastening area that surrounds the opening and contacts the oil pan; and a balance shaft housing that is disposed between the oil pan and the crankcase to cover the opening, wherein the crankcase is provided with plural housing side fastening sections that fasten the crankcase and the balance shaft housing, the fastening section of the plural housing side fastening sections in a first axial direction side of the balance shaft is disposed on an inside of the oil pan side fastening area, and the fastening section of the plural housing side fastening sections in a second axial direction side of the balance shaft, that is opposite from the first axial direction, is disposed on an outside of the oil pan side fastening area; characterized in that a speed changing mechanism is disposed in the second axial direction side in the crankcase, the fastening section of the plural housing side fastening sections on an opposite side from the second axial direction side where the speed changing mechanism is disposed is disposed on the inside of the oil pan side fastening area, and the fastening section of the plural housing side fastening sections in the second axial direction side where the speed changing mechanism is disposed is disposed on the outside of the oil pan side fastening area.