A carbon dioxide supply device includes an accumulation unit, an accumulation flow path, a supply flow path, and a control unit. The accumulation unit accumulates carbon dioxide included in an exhaust gas produced by heating in a greenhouse for cultivating plants. The accumulation flow path guides the exhaust gas to the accumulation unit. The supply flow path guides air to the greenhouse via the accumulation unit. The control unit executes accumulation control and supply control. In the accumulation control, carbon dioxide included in the exhaust gas produced by heating during a heating period is accumulated in the accumulation unit. In the supply control, air is guided through the supply flow path during a non-heating period, and carbon dioxide accumulated in the accumulation unit is supplied to the greenhouse. Further, during the heating period, if a start condition is satisfied during the accumulation control, a refill process is executed wherein the supply control and the accumulation control are executed alternately.
