PURPOSE: To efficiently obtain a surely welded filter, in a preparating method for superposing the flanges of an upper plastic case and a lower case having a connection pipe extending obliquely in downward direction provided on the side wall thereof before welding, by specifically providing the aforementioned connection pipe.

CONSTITUTION: A metal net filter material 24 is welded to the inner wall of an upper plastic case having an outer peripheral flange 21 and an opening 22. A lower plastic case 29 having similarly an outer peripheral flange 25 and an opening 27 provided on the side wall 26 thereof and having a connection pipe 28 extending to obliquely in downward direction provided to said opening 27 is prepared. In this case, a part of the root of the pipe 28 is connected to the outer peripheral end of the flange 25 and the flange 25a positioned in the pipe 28 is faced toward the outside of the pipe 28. The surfaces of the flanges 21, 25 of the upper and lower cases are contacted and the case 29 of this filter assembly is placed on a ring shaped support jig 30 while a ring shaped fixing jig 31 is placed on the flange 21. In the next step, if the jig 31 is vibrated by a vibration apparatus while the jigs 30, 31 are pressurized toward a direction shown by the arrow, the contact surfaces of the flanges 21, 25 can be welded surely and efficiently.