PURPOSE: To extend filtration life by supplying an alkali solution to a matrix part to bring said part to an immersion state and subsequently passing through the alkali solution.

CONSTITUTION: The water remaining in a main body 13 is drained from a valve V7 and, thereafter, an alkali solution is sent into the main body 13 from an alkali solution storage tank 2 by a liquid supply pump P through a valve V1 to bring a matrix 14 to an immersion state. Air is introduced into the lower part of the main body 13 from an air compressor 1 through a valve V8 to perform aeration. Whereupon, sticky scale becomes easy to be scraped off by the action of the alkali solution and further receives the mechanical vibration accompanied by aeration to be perfectly released from the matrix 14. By this method, liquid washing effect can be enhanced.