To provide an oil filler structure for an internal combustion engine capable of smoothly injecting oil through an oil filler port without increasing cost.

An internal combustion engine is formed such that sprockets 6 and 7 and tied to the end parts of intake and exhaust cam shafts 4 and 5, respectively, arranged in parallel to each other on a cylinder head 1 and a chain guide 9 is provided to make contact with a chain 8 wound around and between the two sprockets 6 and 7 and guide the chain, and an oil filler port 10 is opened to a head cover 2. The oil filler port 10 is opened in a position situated above the chain guide 9 between the intake and exhaust cam shafts 4 and 5 of a head cover 9 and at least partially overlapped, as seen from a plane, with the chain guide 9. Since flying of oil is prevented by the chain guide 9, there is no need to partially cover the oil filler port 10 with a shield plate like a conventional type, the cost is reduced, and oil is smoothly and rapidly injected through the oil filler port 10.