A carbon dioxide supply device that accumulates carbon dioxide included in an exhaust gas produced by combustion for heating a greenhouse for cultivating plants, the device comprising: an accumulation unit that accumulates carbon dioxide included in the exhaust gas; an accumulation flow path that guides the exhaust gas to the accumulation unit; a supply flow path (102, 103) that guides air to the accumulation unit, and guides the air that has passed through the accumulation unit to the greenhouse; a control unit (10, 30, 60-65) that controls flow of the exhaust gas and the air; a bypass flow path that guides part of the air, guided by the supply flow path (102, 103) to the accumulation unit, to the greenhouse without passing through the accumulation unit; and a sensor that detects concentration of carbon dioxide in the green house, the control unit (10, 30, 60-65), during a heating period in which the heating is performed, executing accumulation control in which the exhaust gas produced by the heating is caused to pass through the accumulation unit via the accumulation flow path, so as to accumulate carbon dioxide included in the exhaust gas in the accumulation unit, the control unit (10, 30, 60-65), during a period in which the heating is not performed, executing supply control in which the air is caused to pass through the accumulation unit via the supply flow path (102, 103) and then to flow into the greenhouse, so as to supply carbon dioxide accumulated in the accumulation unit to the greenhouse, and the control unit (10, 30, 60-65) further executing a refill process in which the supply control and the accumulation control are executed alternately when a specified start condition is satisfied during the accumulation control in the heating period, the control unit (10, 30, 60-65), in the supply control, adjusting an amount of the air guided by the bypass flow path so as to adjust the amount of the air passing through the accumulation unit, the control unit (10, 30, 60-65) executing, as the supply control, high-speed supply control and low-speed supply control in which the amount of the air passing through the accumulation unit is smaller than the amount in the high-speed supply control, and the control unit (10, 30, 60-65) executing the high-speed supply control in the refill process whereas the control unit (10, 30, 60-65), during the period in which the heating is not performed, executes the high-speed supply control or the low-speed supply control in accordance with the concentration of carbon dioxide in the greenhouse detected by the sensor. an accumulation unit that accumulates carbon dioxide included in the exhaust gas; an accumulation flow path that guides the exhaust gas to the accumulation unit; a supply flow path (102, 103) that guides air to the accumulation unit, and guides the air that has passed through the accumulation unit to the greenhouse; a control unit (10, 30, 60-65) that controls flow of the exhaust gas and the air; a bypass flow path that guides part of the air, guided by the supply flow path (102, 103) to the accumulation unit, to the greenhouse without passing through the accumulation unit; and a sensor that detects concentration of carbon dioxide in the green house, the control unit (10, 30, 60-65), during a heating period in which the heating is performed, executing accumulation control in which the exhaust gas produced by the heating is caused to pass through the accumulation unit via the accumulation flow path, so as to accumulate carbon dioxide included in the exhaust gas in the accumulation unit, the control unit (10, 30, 60-65), during a period in which the heating is not performed, executing supply control in which the air is caused to pass through the accumulation unit via the supply flow path (102, 103) and then to flow into the greenhouse, so as to supply carbon dioxide accumulated in the accumulation unit to the greenhouse, and the control unit (10, 30, 60-65) further executing a refill process in which the supply control and the accumulation control are executed alternately when a specified start condition is satisfied during the accumulation control in the heating period, the control unit (10, 30, 60-65), in the supply control, adjusting an amount of the air guided by the bypass flow path so as to adjust the amount of the air passing through the accumulation unit, the control unit (10, 30, 60-65) executing, as the supply control, high-speed supply control and low-speed supply control in which the amount of the air passing through the accumulation unit is smaller than the amount in the high-speed supply control, and the control unit (10, 30, 60-65) executing the high-speed supply control in the refill process whereas the control unit (10, 30, 60-65), during the period in which the heating is not performed, executes the high-speed supply control or the low-speed supply control in accordance with the concentration of carbon dioxide in the greenhouse detected by the sensor.