In a porous filter, dust that has been collected from a gas through a filter group installed in a vessel body onto filter surfaces is removed from the filter surfaces by backwashing in which the supply and stoppage of a high-pressure gas ejected toward porous filter elements are repeated by means of a valve operation. Backwash tanks having upstream backwash valves and downstream backwash valves are installed in backwash pipes through which the high-pressure gas is guided from high-pressure-gas supply equipment to backwash nozzles for ejecting the backwash high-pressure gas toward the filter group to form a backwash high-pressure-medium supply line. The outlet pressure of the high-pressure-gas supply equipment is set to “1/critical pressure ratio” or more times a filter inlet gas pressure so that the flow speed of the high-pressure gas ejected from the backwash nozzle is the speed of sound or higher.