A lubricating system for an internal combustion engine includes a recovery pump by which a lubricating oil dropping to and dwelling in a bottom portion of a crankcase after lubricating individual portions of the internal combustion engine is sucked through a pump suction port opened in the bottom portion of the crankcase and is fed to a lubricating oil tank. A supply pump is provided for supplying the lubricating oil from the oil tank to the individual portions of the internal combustion engine. The lubricating tank is integral with the crankcase and is partitioned from a crank chamber by a partition wall projecting from the inside wall of the crankcase. The lubricating system includes an overflow oil passage through which the lubricating oil that flows over the upper edge of a partition wall of the lubricating tank is led to a suction port of the recovery pump.