In a bioreactor, a culture solution derived from a cultivation tank is separated into bacteria cells and filtrate by a bacteria cell filter. The filtrate is introduced from the bacteria cell filter into a micro-nano bubble generation tank where micro-nano-bubbles are mixed with the filtrate. The filtrate containing micro-nano-bubbles is returned to the cultivation tank to activate the microorganisms therein, so that a biological reaction time is reduced by the activated microorganisms.