PURPOSE: To reduce the pressure loss across a filter cloth and prevent the dust collection efficiency from being reduced by a method wherein the change-over from filtration to back washing or vice versa is conducted after maintaining the filter cloth in a static condition free from any positive or negative pressure load for a predetermined length of time.

CONSTITUTION: The static condition without any positive or negative pressure load on the filter cloth for a length of time t0, e.g., aboug 10min is provided at each change-over between back washing t1 and filtration t2 during a dusting period T for a chamber of the forced-type bag filter. Accordingly, even when the dusting operation is repeated, the pressure lost is recovered to a maximum extent for each dusting operation and the effect is not weakened, so that the filter cloth will not be clogged even when being used for a long period of time and the pressure will not be increased with time.