An engine (internal combustion engine), in accordance with an embodiment, includes: a cylinder head disposed above a crankcase; a cylinder head cover attached to cover an upper side of the cylinder head; and an ignition plug provided with a plug part inserted into the cylinder head and a plug cap disposed outside the cylinder head cover. The cylinder head cover includes a gas passage formed in such a U-shape as to surround the plug cap of the ignition plug as viewed from an insertion side of the ignition plug, the gas passage allowing fuel gas (blow-by gas) containing oil to flow in from the crankcase.