PURPOSE: To make an engine compact and also simplify drive train of an auxiliary equipment for engine by connecting co-axially a pump shaft of an air pump and a drive shaft of an auxiliary equipment for engine to each other, and interlocking either one of the pump shaft and the drive shaft with a crankshaft through a single drive means.

CONSTITUTION: Since two auxiliary equipment 56 for an engine including an air pump 70 are positioned in a space 20 enclosed by the air pump 70, the other auxiliary equipment 56 for the engine, cylinders, and intake passages 14 and 17 in parallel to a crankshaft 2 in axial direction, they can be arranged compact with ease in a restricted space 20 around the cylinders as compared with the case in which they are distributed for arrangement around the cylinders and crankcase 3. In addition, since a pump shaft 74 of the air pump 70 and a drive shaft of the auxiliary equipment 56 for the engine are driven co-axially with each other, the engine is driven by just connecting either one of the pump shaft 74 and drive shaft to the crankshaft 2, whereby the number of shafts to be driven can be reduced.