An internal combustion engine oil supply system contained in a body of the engine, in which a pump casing common to first and second oil pumps having a common pump shaft rotated interlockingly with rotation of a crankshaft of the engine is provided with a main suction port common to the first and second oil pumps, and first and second relief valves are individually connected to discharge passages of the first and second oil pumps. The main suction port is partitioned into first and second suction ports. The pump casing is formed with a first suction passage extending from the first suction port to a suction side of the first oil pump, and a second suction passage extending from the second suction port to a suction side of the second oil pump. Release ports of the first and second relief valves are connected to the second suction passage.