An ozonized gas having a pressure exceeding atmospheric pressure and having a predetermined concentration is supplied to adsorption/desorption columns (4) at a low temperature state of 0° C. or less and a high pressure and packed with silica gel (6) serving as an adsorbent. The adsorption/desorption columns (4) have been constituted so that at least three of a plurality of adsorption/desorption columns (4), i.e., adsorption/desorption columns (4-1, 4-2, and 4-3), are disposed in a serial cycle arrangement to constitute a main adsorption/desorption column group (99), and that an adsorption/desorption column (4-4) is disposed in parallel with the main adsorption/desorption column group (99) to constitute an auxiliary adsorption/desorption column (999). In a period in which none of the three columns of the main adsorption/desorption column group (99) is performing desorption processing, the auxiliary adsorption/desorption column (999) performs desorption processing. Therefore, the apparatus can continuously output high-concentration ozone at a high flow rate. By thus efficiently producing a high-concentration ozonized gas, the amount of the ozone capable of being output can be increased and the amount of discharge gas which is not permitted to be output can be reduced. That efficient production enables the apparatus for producing high-concentration ozone gas to be compact, satisfactorily operable, and inexpensive.