In an exhaust passage, an exhaust manifold connected to exhaust ports of each cylinder of an engine body and formed to extend vertically is arranged in a side portion of a cylinder head. A secondary air supply system has an intake silencer, a check valve configured to permit only a flow to the exhaust passage side, a secondary air supply control valve configured to control a flow rate of the secondary air, and a secondary air supply passage configured to connect those components to each other and the exhaust passage. The secondary air supply system is arranged in a side portion of a cylinder block placed between a crankcase of the engine body and an assembly of the exhaust manifold and the cylinder head.