PURPOSE: To increase an electric effect to an external field and to obtain electric absorption effect for dusts, fine particles, bacteria, etc., by increasing a difference of surface charge densities of front and rear surfaces.

CONSTITUTION: Cloth 6 supplied by a feed roll 5 is so supplied that its one side surface is brought into contact with semiconductor liquid of a surface wetted with a liquid-containing material 4 fed by turn rolls 7, 8, and subjected to an electret processing by a needlelike electrode 10 mounted in a space through a high voltage generator 9. In this case, adhesive properties of a material to be subjected to the electret processing with a ground electrode made of the liquid-containing material are improved to sufficiently inject charge, and a difference of surface charge densities of front and rear surfaces is set to 4×10-10-9×10-10 coulomb/cm2. Thus, it can be made excellent as a dust-proof cap material.