PURPOSE: To obtain a multiple filter hardly deteriorating its filtering characteristics over a long period of time by using a mechanical filter medium on the upper stream side of an electrified filter medium and superposing two or more filter media in total.

CONSTITUTION: A mechanical filter medium 1a such as a nonwoven fabric of glass fibers or PP fibers is disposed on the upper stream side of air and an electrified filter medium 1b such as a nonwoven fabric of electrified PP fibers is disposed on the downstream side and superposed on the mechanical filter medium 1b. When air is allowed to flow, solid matter in the air is mechanically stopped and removed first by the mechanical filter medium 1a and then the air is passed through the electrified filter medium 1b to electrically attract and stop solid matter.