In an engine (internal combustion engine), a crankcase is provided with a fastening area that surrounds the opening and contacts the oil pan, a balance shaft housing is disposed in the crankcase to surround the opening, and four housing side fastening sections are provided to the crankcase to fasten the crankcase and the balance shaft housing. The fastening section of the four housing side fastening sections in a first axial direction side of a balance shaft is disposed on the inside of the fastening area, and the fastening section of the four housing side fastening sections in a second axial direction side of the balance shaft is disposed on the outside of the fastening area.