An inside air control system comprising: a gas supply device configured to produce low oxygen concentration air having a lower oxygen concentration than outside air as a result of removal of oxygen from the outside air, the gas supply device being capable of supplying the low oxygen concentration air and the outside air into a storage box for storing a breathing plant; and a controller configured to control an operation of the gas supply device, characterized in that the inside air control system performing an oxygen concentration reduction action to reduce an oxygen concentration of inside air in the storage box to a predetermined set oxygen concentration through the gas supply device supplying the low oxygen concentration air into the storage box, and an air composition maintenance action to maintain the inside air at a desired composition after termination of the oxygen concentration reduction action, during the air composition maintenance action, the controller being configured to instruct the gas supply device to operate while switching among a first supply operation in which first low oxygen concentration air is supplied into the storage box, a second supply operation in which second low oxygen concentration air having a lower oxygen concentration than the first low oxygen concentration air is supplied into the storage box, an outside air supply operation in which the outside air is supplied into the storage box, and a standby operation in which supply of the low oxygen concentration air and the outside air into the storage box is maintained at rest, so that the oxygen concentration of the inside air will be the set oxygen concentration, and a carbon dioxide concentration of the inside air will be a predetermined set carbon dioxide concentration. a gas supply device configured to produce low oxygen concentration air having a lower oxygen concentration than outside air as a result of removal of oxygen from the outside air, the gas supply device being capable of supplying the low oxygen concentration air and the outside air into a storage box for storing a breathing plant; and a controller configured to control an operation of the gas supply device, characterized in that the inside air control system performing an oxygen concentration reduction action to reduce an oxygen concentration of inside air in the storage box to a predetermined set oxygen concentration through the gas supply device supplying the low oxygen concentration air into the storage box, and an air composition maintenance action to maintain the inside air at a desired composition after termination of the oxygen concentration reduction action, during the air composition maintenance action, the controller being configured to instruct the gas supply device to operate while switching among a first supply operation in which first low oxygen concentration air is supplied into the storage box, a second supply operation in which second low oxygen concentration air having a lower oxygen concentration than the first low oxygen concentration air is supplied into the storage box, an outside air supply operation in which the outside air is supplied into the storage box, and a standby operation in which supply of the low oxygen concentration air and the outside air into the storage box is maintained at rest, so that the oxygen concentration of the inside air will be the set oxygen concentration, and a carbon dioxide concentration of the inside air will be a predetermined set carbon dioxide concentration.