A CARBON DIOXIDE application system that supplies carbon dioxide to gardening plants in an agricultural greenhouse, the system comprising: a carbon dioxide supply device (10, 20) that supplies carbon dioxide; a tube (20, 31) that is formed to extend from the carbon dioxide supply device (10, 20) to a vicinity of a foot of gardening plants and supplies carbon dioxide supplied from the carbon dioxide supply device (10, 20) from an ejection outlet formed in a portion in the vicinity of the foot of gardening plants; a ventilation device that ventilates an inside of the agricultural greenhouse; and a control device (50, 90) that controls the carbon dioxide supply device (10, 20) and the ventilation device, and, when temperature in the agricultural greenhouse reaches a specified value or more, drives the ventilation device, the carbon dioxide application system being characterised in that the control device (50, 90) alternately repeats execution of: supplying in which the control device (50, 90) drives the carbon dioxide supply device (10, 20) for a first-period specified in advance during which the ventilation device is stopped; and spreading in which, after the supplying being executed, the control device (50, 90) stops the carbon dioxide supply device (10, 20) and the ventilation device for a second-period specified in advance, and wherein, when the temperature in the agricultural greenhouse reaches a specified value or more while one of the supplying and the spreading being executed, the control device (50, 90) inhibits the ventilation device from subsequently driving until end of execution of the spreading. a carbon dioxide supply device (10, 20) that supplies carbon dioxide; a tube (20, 31) that is formed to extend from the carbon dioxide supply device (10, 20) to a vicinity of a foot of gardening plants and supplies carbon dioxide supplied from the carbon dioxide supply device (10, 20) from an ejection outlet formed in a portion in the vicinity of the foot of gardening plants; a ventilation device that ventilates an inside of the agricultural greenhouse; and a control device (50, 90) that controls the carbon dioxide supply device (10, 20) and the ventilation device, and, when temperature in the agricultural greenhouse reaches a specified value or more, drives the ventilation device, the carbon dioxide application system being characterised in that the control device (50, 90) alternately repeats execution of: supplying in which the control device (50, 90) drives the carbon dioxide supply device (10, 20) for a first-period specified in advance during which the ventilation device is stopped; and spreading in which, after the supplying being executed, the control device (50, 90) stops the carbon dioxide supply device (10, 20) and the ventilation device for a second-period specified in advance, and wherein, when the temperature in the agricultural greenhouse reaches a specified value or more while one of the supplying and the spreading being executed, the control device (50, 90) inhibits the ventilation device from subsequently driving until end of execution of the spreading. supplying in which the control device (50, 90) drives the carbon dioxide supply device (10, 20) for a first-period specified in advance during which the ventilation device is stopped; and spreading in which, after the supplying being executed, the control device (50, 90) stops the carbon dioxide supply device (10, 20) and the ventilation device for a second-period specified in advance, and wherein, when the temperature in the agricultural greenhouse reaches a specified value or more while one of the supplying and the spreading being executed, the control device (50, 90) inhibits the ventilation device from subsequently driving until end of execution of the spreading.