Lubrication structure for internal combustion engine

A lubrication structure for an internal combustion engine, comprising:

• a crankcase configured by connecting an upper crankcase on a mating surface of a lower crankcase from above, having an oil pan that stores oil and is disposed in a lower part of the lower crankcase, and configuring a main part of the internal combustion engine;
• an oil pump that is installed in the lower crankcase, sucks oil in the oil pan through a suction passage formed in the lower crankcase, and discharges the oil into a discharge passage formed in the lower crankcase to pressure-feed the oil;
• and
• a relief valve that is installed in the lower crankcase, and performs a valve opening operation in accordance with pressure of oil in the discharge passage to return surplus oil in the discharge passage to the suction passage through a surplus passage formed in the lower crankcase,
• wherein a relief valve housing section that houses the relief valve, and the surplus passage that allows the relief valve housing section and the suction passage to communicate with each other are provided to communicate with the mating surface of the lower crankcase directly or indirectly.