Internal combustion engine and straddle-type vehicle including the same

An engine includes a main passage in which a throttle valve is provided, and first and second bypass passages that bypass the throttle valve. The engine further includes a valve device capable of opening and closing the first and second bypass passages. Both of the first and second bypass passages are closed or at least intermittently opened before startup of the engine. Each of the first and second bypass passages is at least intermittently opened during cold idling. One of the first and second bypass passages is closed and the other one of the first and second bypass passages is opened during warm idling.