A lubrication structure for an internal combustion engine includes a crankcase, an oil pump, and a relief valve. The crankcase is configured by connecting an upper crankcase on a mating surface of a lower crankcase, and has an oil pan. The oil pump and the relief valve are installed in the lower crankcase. The oil pump sucks oil in the oil pan through a suction passage, and discharges the oil into a discharge passage. The relief valve performs a valve-opening operation according to pressure of oil in the discharge passage to return surplus oil in the discharge passage to the suction passage through a surplus passage. These passages are formed in the lower crankcase. A relief valve housing section housing the relief valve, and the surplus passage allowing the relief valve housing section and the suction passage to communicate with each other, are provided to communicate with the mating surface.

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