PURPOSE: To provide a method for producing ultra-fine fiber nonwoven fabric good in balance between filtration accuracy and life and suitable for the produc tion of filter cartridges in a good yield.

CONSTITUTION: When nonwoven fabric comprising ultra-fine fibers having an average fiber diameter of 0.1-10μm and an early tensile resistance of 10-30kg/5cm width per METSUKE (weight) of 30-g/m2 is produced from polypropylene by a spun-bonding method or by a melt0blowing method, one or both of air permeability and fiber dispersibility is measured on line, while the nonwoven fabric is wound with a winder, and on the basis of the measurement result, a feedback operation for changing at least one of the temperature of the polymer, the temperature or flow volume of the air, and a distance between the nozzle and a fiber-collecting member is executed to compensate the changes in the nonwoven fabric with time, thus forming the uniform ultrafine nonwoven fabric having a Frazier air permeation degree of 10-50cm3/cm2/sec per METSUKE of 30g/m2 and a transmitted light strength variation coeffrcient of ≤10. The ultra-fine nonwoven fabric is formed into a prescribed shape to obtain a filter cartridge.