Water (23) that contains micro-nano bubbles generated in a micro-nano bubble generation tank (6) is introduced and treated in a charcoal water tank (11) which is filled with a charcoal (15) and in which an air diffusing pipe (12) is placed and thereafter introduced and treated in a membrane device (21). Thus, activities of microorganisms propagating in the charcoal (15) are increased by the micro-nano bubbles, markedly increasing ability of decomposing organic matters in the water. Therefore, a clogging phenomenon due to the organic matters can be prevented by reducing organic loads on the membrane device (21). Moreover, a very small amount of alcohols or salts are added as a micro-nano bubble generation aid to the micro-nano bubble generation tank (6), improving an incidence rate of the micro-nano bubbles. In the case, since the alcohols and salts are easily decomposed by the charcoal water tank (11) and easily removed by the membrane device (21) in the subsequent stage, no bad influence is exerted on the membrane device (21).