pump 11 communicates with the buffer tank 13 and a state where the outlet 11 b of the vacuum pump 11 communicates with the gas discharge passage 6 through a connection passage 16 the gas introduction passage 4 and the concentrated ozone gas derivation passage 5 are joined together at a location that is closer to the adsorbing cylinders 2 than the locations of the gas introduction valve 7 and the gas derivation valve 10 and joint passages 17 thereof are connected to the corresponding adsorbing cylinders 2 the gas discharge passage 6 is connected to the respective adsorbing cylinders 2 through corresponding gas discharge valves 18 an ozone decomposer 19 is disposed in the gas discharge passage 6 and an outlet of this ozone decomposer 19 is connected to the upstream side of the ozone generator 8 in the gas introduction passage 4 so as to communicate with the ozone generator 8 the gas discharge valve 18 attached to each adsorbing cylinder 2 is opened and closed in relation to the open or close operation of the gas introduction valve 7 attached to the adsorbing cylinder 2 that is the same as that to which the gas discharge valve 18 is attached accordingly the gas discharge valve 18 is opened when ozone oxygen mixture gas is supplied to a corresponding one of the adsorbing cylinders 2 so.