To solve such problems that conventional methods of electretizing nonwoven cloth by applying ultrasonic vibration thereto and of corona-charging after treating with high pressure fluid atomization need complicated procedures to cause a high production cost, and that a material is expensive and pleating is difficult in Tribo-charging.

The nonwoven cloth composed of unstretched fibers of a thermoplastic resin is electret-processed at a temperature near its crystallization temperature. The thermoplastic resin is at least one resin selected from the group consisting of a polyolefin, polyethylene terephthalate, polybutylene terephthalate, and a polyamide, wherein the birefringent index of the unstretched fibers is 0-0.08, the basis weight of the nonwoven cloth composed of the unstretched fibers is 20-500 g/m2, and the cloth is processed in pleats.