PURPOSE: To surely backwash a porous ceramic filter by using a cleaning soln. supply means and a mixing means for generating a two-phase current to supply the two-phase current consisting of high-pressure air and the cleaning soln.

CONSTITUTION: A mixing unit 80 is provided with a cleaning soln. inlet 82, an air inlet 84 and a water jet outlet 86. The cleaning soln. is supplied by a cleaning soln. pump 56, introduced from the inlet 82 and passed through a through hole 88 in the X direction. Meanwhile, high-pressure air from an air supply means 60 is supplied from the air inlet 84 in the y direction, and its center line is aligned with that of the hole 88. The air is injected into the hole 88 from a nozzle 90 set on the center line, and a water jet is formed and supplied into a housing 18 from the outlet 86. Consequently, a water jet at a sufficient cleaning pressure is continuously supplied, the filter is excellently cleaned, and the cost is reduced.