PURPOSE: To prevent the deterioration in collection efficiency, by bringing a nonwoven fabric comprising polyolefinic synthetic fibers having inherent viscosity of 1.5 or less into close contact with an earth electrode having a smooth surface at a room temp. to bring the same to an electret state in a high voltage electric field.

CONSTITUTION: The web with a fiber diameter of 12μm is formed of polypropylene fibers with inherent viscosity of 0.8 by a melt blow method and a number of webs are laminated to form a nonwoven fabric with a wt. of 60g/m2 and a thickness of 0.7mm. The gap between a discharge electrode with a diameter of 0.2mm and an earth electrode comprising a steel drum having a smooth surface is set to 15mm and DC voltage of 10kV/cm is applied between two electrodes to form a high electric field through which the nonwoven fabric is, in turn, passed at a room temp. for 5sec while closely contacted with the earth electrode to be brought to an electret state.