PURPOSE: To enhance arresting efficiency and a ventilating degree and increase a suction work rate by jointing an electret-formed nonwoven fabric having a specific single fiber diameter, filling rate and thickness and a nonwoven fabric having specific ventilating degree and bending strength each other by one layer or more, and by manufacturing a bag with fusion-welding.

CONSTITUTION: A filter bag is formed with an electret-formed nonwoven fabric A having a single fiber diameter of 1 - 5 μm, a filling rate of 10 - 30 % and a thickness of 0.05 - 0.5 mm and a nonwoven fabric B with a ventilating degree of 50cc/cm3/sec or higher, jointed each other by at least one layer or more is manufactured by fusion-welding. In this case, for the nonwoven fabric B, the one with the bending strength of 0.3 g fcm3/cm or higher is used. On electret-forming the nonwoven fabric A, for example, the temperature of the nonwoven fabric is heightened to about the softening point temperature, and heat electret or the like is obtained by applying DC high tension to said temperature while cooling. As for material for forming the nonwoven fabric A, polyolefin is desirable in particular, and for the nonwoven fabric B, the nonwoven fabric of a celulose, a polyester, a polyamide, or a polyolefin excellent in fusion welding property is desirable.