PURPOSE: To increase the amount of fine gas bubbles formed during electrolysis, by providing a cell for the electrolysis of sludge, a power source, a tank for the treatment of flocs in floating sludge and a means for separately supplying sludge and an electrolytic solution to be electrolyzed to a space between electrodes.

CONSTITUTION: Fine gas bubbles are efficiently formed by the electrolysis of an electrolytic solution supplied through an inlet 11 for the inflow of an electrolytic solution provided below electrodes 4, so that flocs in sludge supplied through an inlet 1 for the inflow of sludge are let float up by said fine gas bubbles and stay on the surfaces in an electrolytic cell 12 and a separation tank 7. This floating sludge is scratched off by a sludge scraper 6 and dropped into a concn. sludge tank 8. The separated water from which the flocs have been removed is circulated through an outflow device 9 and a device 10 for regulating a water level and then drained or recovered. The separated water recovered through a water intake 16 for separated water is mixed with alkali or the like charged from a hopper 15 in a tank 13 for supplying an electrolytic solution and then supplied through an injection opening 11 to the electrolytic cell 2.