To improve the value and safety of a product provided with a conspicuous swelling in which a core material is not put.

A lower electrode 1 whose upper surface is flat and an upper electrode 2 in an endless ring shape are constituted, the inner side of the entire peripheral lower edge part of the upper electrode 2 is ground to a circular arcuate surface 3 and a welding blade 4 whose cross section is V-shaped is formed at the same part. Thereafter, a back side sheet 5 made of a thermoplastic flexible synthetic resin is piled on the upper surface of the lower electrode 1, a front side sheet 6 made of the thermoplastic flexible synthetic resin is piled on the back side sheet 5 respectively, the upper electrode 2 is pressed to the prescribed part of the front side sheet 6 and a high frequency voltage is applied to both upper and lower electrodes 2 and 1 while pressing. As the upper electrode 2 is lowered by the compression and application, the circular arc surface 3 of the welding blade 4 pushes away the entire outer peripheral part 8 of the part 7 surrounded by the upper electrode 2 on the front side sheet 6 and the center part 9 of the surrounded part 7 is swollen upwards. After the swelling, in the state of the compression and the state of cancelling the application, the solidification of a molten part by the welding blade 4 is waited for, and after the solidification, the upper electrode 2 is raised.