To improve productivity by simultaneously performing desired flare working to plural pipes.

In this method, flare working is applied to a pipe 6 by inserting a flare working part 35 from the tip part of the pipe 6 of a heat exchanger 1 consisting of plural laminated fins 3 and the U-shaped pipe 6 piercing these fins 3. In this case, when plastically deforming the tip part of the pipe 6 by individually inserting the flare working part 35 into the tip parts of the plural pipes 6, by fitting a flare guide pipe 40 in the tip part of the pipe 6, a deformation quantity is controlled by the outside diameter standard of the pipe. A load caused by this control is individually and independently absorbed by a belleville spring 44.