A technique for making air less likely to enter a strainer in an internal combustion engine in which the strainer is supported by a drain bolt. In an internal combustion engine in which a drain bolt supports a strainer and is configured to discharge a drain is attached to a bottom of a sump. A drain hole into which the drain bolt is screwed and a strainer hole are arranged coaxially. A crankcase is provided with an oil passage through which oil is returned to the sump. The oil passage includes a first passage through which the oil flows in a direction away from the strainer, a return passage that turns around from a forefront end of the first passage, and a second passage through which the oil flows from a forefront end of the return passage toward the strainer.