To reduce the number of components and a size of a whole engine, facilitate mounting to vehicle and the like, and increase flexibility in setting a change gear ratio of an air pump, in an air pump drive device which drive the air pump with an engine.

The engine is constituted to be 4-cycle having a camshaft 41 constituting a part of a valve system 38 and arranged to a cylinder block 13. A pump case 63 of the air pump 61 to which a power is transmitted from the camshaft 41 is integrally formed to the cylinder block 13.