To provide a method for separating to acquire a high-purity oxygen gas from air or the like with a PSA gas separation apparatus and capable of separating to acquire a high-purity nitrogen gas continuously and efficiently from a desorption gas supplied continuously from the PSA gas separation apparatus, and a system.

A PSA gas separation process in the PSA gas separation apparatus 1 and a membrane type gas separation process in a membrane type gas separator 2 are included. In the PSA gas separation process, an oxygen enriched gas, and the desorption gas containing mainly nitrogen and also oxygen are removed from oxygen- and nitrogen-containing gas such as air by a pressure fluctuation adsorption type gas separation method to be performed using an adsorption column filled with an adsorbent adsorbing nitrogen on a priority base. In the membrane type gas separation process, the desorption gas is separated into gas passing through a gas separation membrane 2A and gas not passing therethrough (nitrogen enrichment gas) by the gas separation membrane 2A while the pressure of the passing side of the gas separation membrane 2A through which oxygen is passed on a priority base is reduced at less than an atmospheric pressure.