A greenhouse manager would supply CO₂ to the greenhouse, even in a highly ventilated greenhouse, if it were certainly cost effective to supply CO₂ at a certain rate. However, for a ventilated greenhouse, no useful index or criterion for the assessment of CO₂ supply cost effectiveness. We propose a response curve of the canopy net photosynthetic rate (CNPR) to the CO₂ supply rate (CSR) for a greenhouse on a cost-effectiveness assessment basis, with a novel procedure for real-time estimation of the response curve of CNPR to CSR in a ventilated greenhouse (CSR-CNPR curve). A greenhouse manager, based on a CSR-CNPR curve created with a short interval (a few minutes), can control the CSR to a greenhouse using a computerized greenhouse environment control system. To create a real-time CSR-CNPR curve for a short interval, PPFD, RH, temperature, CO₂ concentration inside and outside the greenhouse, the number of air exchanges per hour of the greenhouse, CNPR during the day, and canopy dark respiration rate during the night must be recorded, as the first procedure, for a short interval of more than 10 days after plants are transplanted into a greenhouse. (modified from Fujiwara and Oshima (2017): Abstract Book of GreenSys2017)