In an internal combustion engine, a first oil chamber includes a curved surface portion provided on the upstream side in an oil flow direction and an inclined surface portion provided on the downstream side. The engine is so constructed that oil drops from upper oil passages onto the curved surface portion on the upstream side of the first oil chamber and the inclined surface portion on the downstream side. The curved surface portion on the upstream side has a curved shape which is convex downward, and the inclined surface portion on the downstream side has a slope shape which is inclined downward with respect to the horizontal direction. The curved shape of the curved surface portion on the upstream side is connected to the inclined surface portion before a tangent line of the curved surface portion turns to the horizontal direction.