PURPOSE: To make electric charge dielectric on the surface of a dust collection filter by equipping an electrode part constituted of a high voltage electrode and a pair of counter electrodes, the dust collection filter and a high voltage impressing part.

CONSTITUTION: When polluted air is passed through an electrode part 3, dust contained in the air is subjected to plasma charge with positive electric charge by discharge emitted from an acicular projection 5a. When the air contg. charged dust is passed through an earthed dust collection filter 6, dust is collected by this dust collection filter 6. Only the cleaned air is sent to the outside of a main body from a blowout port. Negative electric charge is made dielectric on the surface of the earthed dust collection filter 6. Dust is stuck on the surface of the dust collection filter 6 and electric potential is leaned to the positive side. The phenomena are prevented wherein potential difference of the charged dust is lowered and the collection capacity of the dust collection filter 6 is deteriorated. Thereby the collection efficiency is maintained for a long period.