To enable air to permeate even the interior of a spacer and thereby improve the pressure loss and the wind velocity of an entire filter by forming the spacer of a foamed resin, in a filter bag consisting of a filter medium folded in a zigzag fashion and a spacer attached to the surface of the filter medium.

A filter medium 1 is drawn out of a roll 11 and then is folded in a zigzag fashion by a folding part 12, and further, the folded filter medium 1 is heated at a heating part 13. In addition, the folded filter medium 1 is once developed by a development part 14 and a molten spacer material is discharged linearly from a nozzle 16 at a spacer application part 15 to attach spacers in a specified pattern. In this case, a resin is made to foam with the help of a foaming agent. Further, the filter medium 1 is again folded in a zigzag fashion at a rising part 17 to complete a mini-pleats air filter bag. Thus it is possible to make air permeate the interior of the spacer and reduce the pressure loss of the entire filter and at the same time, control a turbulence in the air stream near the spacer and make the wind velocity distribution of the entire filter uniform.