An air composition adjustment apparatus adjusts the composition of inside air inside a storage. The air composition adjustment apparatus includes an air processor, an air supply, a detector, and a controller. An air processor supplies a treated gas having a composition different from that of outside air to the storage. The air supply supplies the outside air to the storage. The detector detects a concentration of a component of the inside air. The controller adjusts a flow rate of the outside air supplied to the storage by the air supply based on a value detected by the detector.