1. An internal combustion engine comprising: a crankshaft; a starter motor including a motor shaft; a starter gear fixed to the crankshaft, the starter gear receiving a driving force from the motor shaft; a one-way clutch that is interposed between the crankshaft and the starter gear, the one-way clutch being configured to transmit a driving force from the starter gear to the crankshaft, and to block a driving force from the crankshaft to the starter gear; and a balancer, including a balancer drive gear fixed to the crankshaft, a balancer driven gear that is driven by the balancer drive gear to thereby rotate, wherein the one-way clutch and the balancer drive gear are formed integrally, wherein: the one-way clutch includes a cylindrical inner race, a cylindrical outer race arranged radially outward of the inner race, and a clutch element arranged between the inner race and the outer race; the outer race is fixed to the crankshaft; the inner race is formed integrally with the starter gear; and the balancer drive gear includes gear teeth formed integrally with the outer race, and wherein the gear teeth are formed on a radially outer portion of the outer race.