A ventilation device to be installed outdoors, includes: an air supply fan; an exhaust fan; a heat exchanger; and a casing having an accommodation space that accommodates the air supply fan, the exhaust fan, and the heat exchanger. The casing includes: a partition defining a lower end of the accommodation space and including a first outside air intake port and a first exhaust port each passing through the partition in an up-and-down direction; a bottom plate; an outside air intake chamber defined between the bottom plate and the partition disposed above the bottom plate and that communicates with the first outside air intake port; an exhaust chamber defined between the bottom plate and the partition and that communicates with the first exhaust port; and side plates.