PURPOSE: To keep constant particulate removal performance irrespective of the kind of a liq. to be filtered by injecting an inonic gas into the liq. to be filtered and then filtering the liq.

CONSTITUTION: The removal performance of a fluororesin filter is closely related to the resistivity of a liq. to be filtered. Namely, the particulate in a liq. low in resistivity is highly removed, and the removal of in the liq. high in resistivity is poor. Accordingly, an ionic gas is injected into the liq. to be treated, and the liq. is then filtered. Although any gas dissolving in the liq. to form an ion can be used as the ionic gas, gaseous carbon dioxide, ammonia, sulfur dioxide, etc., are exemplified, and gaseous carbon dioxide capable of being removed after filtration is most preferably used. The amt. of the ionic gas to be injected depends on the kind of liq. and ionic gas, and the resistivity of the liq. charged with the gas is controlled to ≤200kΩ.cm.