An internal combustion engine comprising: a crankshaft; a clutch configured to receive torque from the crankshaft; a cam chain interlocking with the crankshaft; an upper crankcase disposed above the crankshaft; a lower crankcase disposed below the crankshaft and joined to the upper crankcase; the upper crankcase and the lower crankcase including a crank chamber accommodating the crankshaft, a clutch chamber accommodating the clutch and being in communication with the crank chamber, and a cam chain chamber accommodating a portion of the cam chain; the upper crankcase including a first upper partition wall separating the cam chain chamber and the crank chamber from each other, and a second upper partition wall including a bottom surface and separating the cam chain chamber and the clutch chamber from each other; the lower crankcase including a first lower partition wall being in contact with the first upper partition wall and separating the cam chain chamber and the crank chamber from each other, and a second lower partition wall including a top surface in contact with the bottom surface of the second upper partition wall and separating the cam chain chamber and the clutch chamber from each other; an oil passage provided in the first lower partition wall and configured to allow communication between the cam chain chamber and the crank chamber and to guide oil from the cam chain chamber to the crank chamber; an oil pan disposed below the lower crankcase and configured to recover oil from the crank chamber; and a first passage (53,63) including a groove provided in at least one of the bottom surface of the second upper partition wall and the top surface of the second lower partition wall, and configured to allow communication between the cam chain chamber and the clutch chamber. a crankshaft; a clutch configured to receive torque from the crankshaft; a cam chain interlocking with the crankshaft; an upper crankcase disposed above the crankshaft; a lower crankcase disposed below the crankshaft and joined to the upper crankcase; the upper crankcase and the lower crankcase including a crank chamber accommodating the crankshaft, a clutch chamber accommodating the clutch and being in communication with the crank chamber, and a cam chain chamber accommodating a portion of the cam chain; the upper crankcase including a first upper partition wall separating the cam chain chamber and the crank chamber from each other, and a second upper partition wall including a bottom surface and separating the cam chain chamber and the clutch chamber from each other; the lower crankcase including a first lower partition wall being in contact with the first upper partition wall and separating the cam chain chamber and the crank chamber from each other, and a second lower partition wall including a top surface in contact with the bottom surface of the second upper partition wall and separating the cam chain chamber and the clutch chamber from each other; an oil passage provided in the first lower partition wall and configured to allow communication between the cam chain chamber and the crank chamber and to guide oil from the cam chain chamber to the crank chamber; an oil pan disposed below the lower crankcase and configured to recover oil from the crank chamber; and a first passage (53,63) including a groove provided in at least one of the bottom surface of the second upper partition wall and the top surface of the second lower partition wall, and configured to allow communication between the cam chain chamber and the clutch chamber.