A oil supply structure of an internal combustion engine, includes: an oil flowing path which is formed to extend along a camshaft vertically above at least one of intake and exhaust camshafts (3a, 3b) of the internal combustion engine, and in which oil flows; an oil supply portion provided on one end side of the oil flowing path, and supplying oil to the oil flowing path; and oil ejection holes aligned along the oil flowing path, provided to be opened vertically downward, and through which the oil of the oil flowing path is ejected toward the camshaft, wherein, in the oil flowing path, a partitioning portion, which extends along the oil ejection holes toward the other end side opposite to the oil supply portion from one end side, and partitions the inside of the oil flowing path, is provided.