There is provided an engine that does not require additional processing at the time of component mounting, and component mounting work of which is also easy. The engine includes an oil gallery 3, a base attaching seat 5 provided in an engine frame wall 4, and a base for component mounting 6 attached to the base attaching seat 5, wherein an oil cooler 1 and an oil filter 2 are mounted on the base for component mounting 6, and the base attaching seat 5 is provided with an oil inlet 5a communicating with an upstream-side passage 3a of the oil gallery 3, and an oil outlet 5b communicating with a downstream-side passage 3b of the oil gallery 3, and by attaching the base for component mounting 6 to the base attaching seat 5, engine oil 7 flowing into the oil inlet 5a of the base attaching seat 5from the upstream-side passage 3a of the oil gallery 3 passes the base for component mounting 6, the oil cooler 1, and the oil filter 2, and flows out of the oil outlet 5b of the base attaching seat5 to the downstream-side passage 3b of the oil gallery 3.