A collecting exhaust port 18 provided in a cylinder head 12 is comprised of exhaust port sections 46 extending from exhaust valve bores 35 in cylinders 14, and an exhaust collecting section 47 in which the exhaust port sections 46 are collected. The cylinder head 12 includes a protrusion 49 projecting in an arch shape outside a side wall 11 1 of a cylinder block 11. Oil passages 55 1 , 55 2 are defined in regions surrounded by the exhaust collecting section 47 and a pair of the exhaust port sections 46 extending from adjacent combustion chambers 16.
