Internal combustion engine

An internal combustion engine comprising:

• a crankshaft supported by a crankcase via a bearing;
• a balancer drive gear disposed on the crankshaft;
• a balancer shaft on which a balancer is disposed;
• a balancer driven gear disposed on the balancer shaft, the balancer driven gear meshing with the balancer drive gear;
• and
• a one-way clutch mechanism that transmits to the crankshaft a drive force to start the internal combustion engine, the balancer drive gear being disposed between the bearing and the one-way clutch mechanism on the crankshaft, wherein
• the balancer drive clear is disposed adjacent to the bearing on an outside of a crank chamber of the crankcase,
• the one-way clutch mechanism includes a case fixed to the crankshaft and a starter driven gear provided relatively rotatably with respect to the case and driven by the drive force,
• the bearing, the balancer drive gear, the starter driven gear, and the case are disposed on the crankshaft in sequence from a side of the crank chamber, side-by-side in an axial direction of the crankshaft, and
• the starter driven gear includes a thrust bearing portion that abuts on the balancer drive gear on a surface, of the starter driven gear, facing the balancer drive gear, and the thrust bearing portion is a protrusion that protrudes in the axial direction of the crankshaft toward the balancer drive gear.