A ventilation device, comprising: a ventilator comprising a heat exchanger that performs heat exchange between exhausted indoor air and supplied external air; and a humidifier, comprising: a first water supply tank provided at a first height and configured to store fluid therein; a second water supply tank provided at a second height that is different from the first height and configured to store fluid therein; a first supply pipe that connects the first and second water supply tanks (320 and 330); a wicking device that absorbs fluid from the first water supply tank and the second water supply tank so as to humidify supplied air passing therethrough from the ventilator; a first fluid level sensor provided in the first supply tank for sensing a level of the fluid in the first supply tank; a second fluid level sensor provided in the second supply tank for sensing a level of the fluid in the second supply tank; a second supply pipe configured to connect the first water supply tank to an external fluid source; a supply valve provided in the second supply pipe so as to selectively control a supply of fluid to the first water supply tank; wherein the first height is higher than the second height and fluid supplied to the first water supply tank by the second supply pipe in excess of a preset level is supplied from the first water supply tank to the second water supply tank via the first supply pipe; wherein the supply valve selectively opens or closes the second supply pipe based on the level of the fluid is the first supply tank or the second supply tank sensed by the first fluid level sensor and the second fluid level sensor. a ventilator comprising a heat exchanger that performs heat exchange between exhausted indoor air and supplied external air; and a humidifier, comprising: a first water supply tank provided at a first height and configured to store fluid therein; a second water supply tank provided at a second height that is different from the first height and configured to store fluid therein; a first supply pipe that connects the first and second water supply tanks (320 and 330); a wicking device that absorbs fluid from the first water supply tank and the second water supply tank so as to humidify supplied air passing therethrough from the ventilator; a first fluid level sensor provided in the first supply tank for sensing a level of the fluid in the first supply tank; a second fluid level sensor provided in the second supply tank for sensing a level of the fluid in the second supply tank; a second supply pipe configured to connect the first water supply tank to an external fluid source; a supply valve provided in the second supply pipe so as to selectively control a supply of fluid to the first water supply tank; wherein the first height is higher than the second height and fluid supplied to the first water supply tank by the second supply pipe in excess of a preset level is supplied from the first water supply tank to the second water supply tank via the first supply pipe; wherein the supply valve selectively opens or closes the second supply pipe based on the level of the fluid is the first supply tank or the second supply tank sensed by the first fluid level sensor and the second fluid level sensor. a first water supply tank provided at a first height and configured to store fluid therein; a second water supply tank provided at a second height that is different from the first height and configured to store fluid therein; a first supply pipe that connects the first and second water supply tanks (320 and 330); a wicking device that absorbs fluid from the first water supply tank and the second water supply tank so as to humidify supplied air passing therethrough from the ventilator; a first fluid level sensor provided in the first supply tank for sensing a level of the fluid in the first supply tank; a second fluid level sensor provided in the second supply tank for sensing a level of the fluid in the second supply tank; a second supply pipe configured to connect the first water supply tank to an external fluid source; a supply valve provided in the second supply pipe so as to selectively control a supply of fluid to the first water supply tank; wherein the first height is higher than the second height and fluid supplied to the first water supply tank by the second supply pipe in excess of a preset level is supplied from the first water supply tank to the second water supply tank via the first supply pipe; wherein the supply valve selectively opens or closes the second supply pipe based on the level of the fluid is the first supply tank or the second supply tank sensed by the first fluid level sensor and the second fluid level sensor.