Water vapor is mixed to O3 gas generated by an ozone generator of discharge type. The mixed fluid is cooled by a cooler, thereby impurities such as metals and nitrogen oxides contained in the O3 gas dissolve into condensed water. Subsequently, a gas-liquid separator separates the O3 gas from the condensed water. Water vapor is mixed with the O3 gas again. The mixed fluid passes through a metal trap composed of a container containing plural silicon chips as a metal adsorbent, thereby to remove the remaining metals therefrom.