The apparatus comprises an elongate die 11 having a plurality of orifices 32 formed in a die tip 30 thereof, an extruder 10 for extruding molten thermoplastic resin through the die tip orifices 32 to form a plurality of side-by-side fibers, and air lines 17 for blowing converging air streams on the fiber extruded from the die tip orifices 32 to form a fiber-air stream 40. A pair of elongate and horizontal electrodes 42, 43 is spaced from the outlets of the die tip orifices 32, one electrode 42 being above the fiber-air stream 40 and the other electrode 43 being below the fiber-air stream 40. A pair of emitters 44 is secured to the die, and protrudes outwardly therefrom, one emitter 44 being positioned above the die tip 30 and the other emitter 44 being positioned below the die tip 30. Each emitter 44 has an outer end 45 in close proximity to the die tip orifice outlets. Furthermore a voltage source is provided for applying a high voltage to the electrodes 42, 43 to establish an electrostatic field between the electrodes 42, 43 and the emitters 44. A filter of exceptional filtration efficiency may be produced by such apparatus.