The present invention provides a method that allows solid-liquid separation of activated sludge and a treated liquid to be carried out stably and efficiently by suitably evaluating the risk of clogging prior to membrane clogging and adopting necessary and adequate countermeasures. The present invention provides a wastewater treatment method employing a membrane separation activated sludge process, by which the upper limit of BOD-sludge load is determined based on an index indicating the total organic content of the organic wastewater and a BOD value, and the BOD-sludge load in the activated sludge tank is adjusted so as not to exceed the upper limit.