PURPOSE: To uniformize fiber density as well as to make dust collecting capacity constant, by applying mechanical treatment to a non-woven fabric comprising a synthetic fiber subjected to antistatic treatment to mutually entangle fibers.

CONSTITUTION: To a non-woven fabric comprising a synthetic fiber made of polypropylene, antistatic treatment by adhering a resin thereto is applied. In the next step, to the treated nonwoven fabric, mechanical treatment such as needle punching is applied to mutually entangle fibers to prepare an air filter. This filter is low in deterioration by the elapse of time and the life thereof can be prolonged to a large extent.