PURPOSE: To prevent pressure fluctuation of compression air discharged from an air pump by smoothing rotation of a pump shaft by using a balancer shaft.

CONSTITUTION: An engine is provided with an air pump 52 to be a supply source of compression air to be injected to a combustion chamber, and a balancer shaft 15 to offset engine vibration by rotating by interlock with a crank shaft 5. An engagement type wrapping connector driving body 70 is applied between a pump shaft 54 of the air pump 52, the crank shaft 5, and the balancer shaft 15, so the balancer shaft 15 and the pump shaft 54 are simultaneously rotated and driven by the wrapping connector driving body 70, and the balancer shaft 15 is disposed to a loosening side 70b of the wrapping connection body 70 to the pump shaft 54.