To dissolve and hold, in water, oxygen gas which is hardly dissolved or reactive gas which contains oxygen and ozone by preparing it to an ultrafine air-bubbling state.

A device feeds oxygen to microorganisms by forming a gasification solution dispersing the reactive gas containing oxygen, or oxygen and ozone as an ultra fine air-bubbling state in a return sludge water or clean water in the outside of a vessel (pond) in a wastewater treating system, and by feeding the gasification solution to a reaction vessel (aerobic or anaerobic), or alternatively, feeding the gasification solution to a vessel before or after a reaction process. Further, the kind, the concentration, the volume, the return vessel (pond) and the feed time of the reactive gas corresponding to the progress state of treating wastewater are decided and are centralizedly controlled.