In an internal combustion engine, upper and lower crankcases include a crank chamber, a clutch chamber in communication with the crank chamber, and a cam chain chamber. The upper crankcase includes a second upper partition wall configured to separate the cam chain chamber and the clutch chamber from each other. The lower crankcase includes a first lower partition wall configured to separate the cam chain chamber and the crank chamber from each other, and a second lower partition wall configured to separate the cam chain chamber and the clutch chamber from each other. An oil passage allowing communication between the cam chain chamber and the crank chamber is provided in the first lower partition wall. First passages allowing communication between the cam chain chamber and the clutch chamber are provided respectively in the bottom surface of the second upper partition wall and in the top surface of the second lower partition wall.