A ventilation outlet of a generation unit case installed outdoors and housing a generation unit including a generator and an internal combustion engine to drive the generator. The outlet comprise a maintenance box whose inside surfaces are attached with sound insulators and bored with an inlet hole at its upper portion to be communicated with an inside of the generation unit case and bored with an outlet hole at its lower portion that connects the inlet hole to exterior through a passage formed among the sound insulators. A flow guide plate is installed near the outlet hole to guide rainwater entered from outside to flow upward and a shield is installed at the passage to prevent the rainwater from flowing further upward.