PURPOSE: To obtain treatment capacity stable over a long period of time, by introducing org. waste water into a biological reaction tank in which a plurality of separation membrane modules are vertically provided and blowing oxygen- containing gas into the lower parts of the modules to perform aerobic treatment.

CONSTITUTION: Org. waste water A is supplied into a biological reaction tank 1 from an introducing pipe 13 by a supply pump 12 and mixed with stagnant sludge to form a liquid B to be treated which is, in turn, subjected to biological metabolic reaction treatment. The supply amount of org. waste water A is controlled so that the liquid level of the liquid B to be treated immerse a separation membrane tube group 2 and is set to a position forming a definite gas stagnant part to the upper part of the reaction tank 1. The liquid B to be treated receives the supply of oxygen from compressed oxygen-containing gas D blown in from the gas diffusion pipe 15 provided to the lower part of the reaction tank 11 and a recirculation stream by air lift effect is generated and the liquid B to be treated is separated by a separation membrane 5 to be transmitted to a transmitted liquid chamber 11.