The CO₂ balance of a closed-type transplant production system with artificial lighting was investigated. Sweetpotato (Ipomoea batatas (L.) Lam cv. Beniazuma) plants were grown for 15 days at photosynthetic photon flux on the tray surface of 100, 200 and 300 μmol m^-2 s^-1 on days 0-2, 3-11 and 12-14, air temperature of 30°C, CO₂ concentration of 1000μmol mol^-1, relative humidity of 80/100% (photoperiod/dark period) inside the system and photoperiod of 16h d^-1. The CO₂ concentration inside the system during dark period was increased with time because of the respiration of plants. The daily amount of supplied CO₂ on day 14 was 10 times larger than on day 1 due to the increasing in net photosynthesis of the plants with passage of days. The amounts of supplied CO₂, assimilated CO₂ by the plants through net photosynthesis and released CO₂ by ventilation to the outside during 15 days were 6.1, 5.6 and O.5 mol m^-2, respectively. These results show that the CO₂ utilization efficiency is more than 7-30 times higher in the closed type transplant production system than in the open type system (i. e. greenhouse), and the closed-type transplant production system is profitable for minimizing the amount of supplied CO₂ in plant/transplant production.