a method of concentrating ozone gas including : causing ozone gas contained in ozone - oxygen mixture gas to be selectively adsorbed to adsorbents which are filled in a non - cooled state in at least two adsorbing cylinders arranged parallel to one another ; desorbing the ozone gas from the adsorbents by subjecting a depressurizing process to each adsorbing cylinder during an ozone gas desorption operation ; repeating an adsorption step and a desorption step alternately in the at least two adsorbing cylinders ; and controlling the adsorbing cylinders , in such a way that one of the adsorbing cylinders is performing the adsorption step while another one of the adsorbing cylinders is performing the desorption step .