An air composition adjusting device includes: an oxygen separator that separates oxygen from external air to be supplied to a target space; a gas supply path including a high concentration gas supply path for oxygen through which the oxygen separator communicates with the target space; and a controller that performs an oxygen concentration raising operation of supplying a high oxygen concentration gas, which has a higher oxygen concentration than external air before being treated by the oxygen separator, to the target space through the high concentration gas supply path for oxygen.