Membrane device having means for charging the membranes

A membrane apparatus (1) includes a bubble supplying portion (4,401) for supplying micro bubbles which is provided at a location in the vicinity of the entrance (110) of a flow passage for raw liquid. Thus, the growth of a gel layer on a membrane surface can be prevented, and a large filtering amount can be maintained while preventing sludge from blocking the membrane (104). Moreover, since bubbles uniformly pass through the inter-membrane passages (108), sludge blocking can be evenly prevented over the entire surface of the membrane (104). When a flat membrane (104) is used as the filtering membrane, there is provided an advantage of reducing the frequency of work for disassembling the frame (101) and cleaning the membranes. The membrane apparatus (1) is connected to an aeration apparatus, a circulating tank (3), and a filtrate pump (2). Since the relative density of raw liquid which has received bubbles in the membrane apparatus decreases, a difference in density is produced between that raw liquid and raw liquid which is newly supplied from the circulating tank to the passage and which contain no bubbles. Since this difference in density causes a flow of raw liquid, no power is needed for circulation. Accordingly, a circulating pump can be omitted so as to reduce costs including initial cost and operation cost.