A ventilator includes an exhaust air duct for exhausting air from an interior space to an outside and a supply air duct for supplying air from the outside to the interior space, and is equipped with a heat exchanger and a defroster. The heat exchanger performs heat exchange between air passing through the exhaust air duct and air passing through the supply air duct. The defroster defrosts, when frost attaches to the heat exchanger, the heat exchanger by, after air flows into the exhaust air duct or the supply air duct, changing a state of the air.