An internal combustion engine is configured so that: a cam chain drive sprocket is provided on a camshaft; one end of a cam chain tensioner guide is pivotally supported on a crank case by a support shaft; and a partition wall member for separating a generator chamber and an inner chain chamber, which is formed within the crankcase so as to accommodate the cam chain drive sprocket, is mounted to the crankcase. The support shaft (59) is disposed outside the partition wall member (66) when viewed in the direction of the axis of a crankshaft (13). The support shaft (59) is inserted from the outside of the crankcase (14) in the direction of the axis of the crankshaft (13) and is supported by the crankcase (14). As a result of this configuration, the partition wall member for separating the inner chain chamber and the generator chamber is compact.