To enable stable supply of ozone gas of constant concentration, by connecting a dilute gas-introducing passage to transfer passage of ozone gas eliminated from a ozone adsorbent, detecting the ozone gas concentration of ozone gas transfer passage and controlling the dilute gas flow rate from the dilute gas introducing passage.

An ozone-oxygen mixed gas obtained by ozonizing oxygen gas led from a raw material oxygen feed source 11 with an ozone generator 9 is passed through a flow rate controller 10 and an ozone feed passage 5 and absorbed into an ozone adsorber 2 in an ozone storage container 1 cooled with a freezing mixture 4 stored in an outer box 3. Then, ozone gas eliminated from the ozone adsorbent 2 is led from an ozone leading passage 6 and ozone gas concentration is detected by an ozone monitor 27. A dilute gas flow rate from a dilute gas-leading passage 24 connected to the ozone leading passage 6 is controlled by mass flow rate controller 26 based on the concentration and discharged through an ozone consumption facility 13, a suction passage 14 and an ozone decomposer 15 to a vacuum pump 19.