An internal combustion engine includes a reduction gear including: at one end thereof, a small-diameter gear in mesh with a one-way clutch gear; and at an opposite end thereof, a large-diameter gear in mesh with a drive gear of a starter motor. The starter motor has a driveshaft disposed below a rotation axis of the reduction gear and within a width of the large-diameter gear as viewed in an axial direction. Accordingly, in the internal combustion engine, it is possible to efficiently dispose components in a reentrant space formed between a crankcase and a cylinder block.