A radiator chamber is provided in a top compartment of a package, whereas an engine is provided in a bottom compartment. Outside the package, a battery unit is attached to an external wall panel (e.g., lower panel) of the bottom compartment. The battery unit includes a dedicated unit ventilation fan and a unit ventilating section that introduces outside air into an enclosure of the battery unit. The enclosure has a lower portion thereof divided to accommodate a package ventilating section that ventilates the bottom compartment of the package. There is provided a detachable bottom compartment inspection window in the external wall panel below the battery unit.