To provide a carbon dioxide applying device capable of increasing a carbon dioxide concentration in a cluster of cultivation crops to be cultivated in an opening space in horticultural facilities.

This carbon dioxide applying device has a combustion carbon dioxide generator 2 which generates carbon dioxide, a blower 10 which is set on the carbon dioxide generator 2, a duct 3 which is connected to the blower 10 and arranged in an appropriate place, a watering tube 4 which is connected to the duct 3, and arranged in a cluster of the cultivation crops 5 in the horticultural facilities, and jets carbon dioxide from a plurality of holes formed on a main body.