PURPOSE: To allow the efficient execution of the peeling of impurities from a filter membrane by relatively oscillating the filter membrane and liquid at the time of passing liquid reverse from the liquid flow at the time of filtering in the method for filtering the liquid by permeating the same through the filter membrane.

CONSTITUTION: A washing liquid is passed under a high pressure from a filtrate port 16 and a concd. liquid port 14 and an exciting device 17 is driven at the time of back washing in the method for filtering the liquid by permeating the same through the filter membrane 19. An oscillating body 22 is oscillated in this way, by which the filter membrane 19 is oscillated. The impurity particles sticking to the filter membrane 19 are dislodged and liberated from the membrane surface by the oscillation of the filter membrane 19. The impurities are then stripped from the filter membrane 19 by the washing liquid flowing in the direction reverse from the flow direction at the time of the filtration and are thereby removed. As a result, the peeling of the impurities from the filter membrane is effectively executed even in the case of filtering liquid having a silica component at a high concn.