PURPOSE: To improve a commodity value by making the appearance fine, by a method wherein a continuous or discontinuous protrusion having fixed size is provided on a fitting surface of a binding tool to the front material of a file made of resin and the protrusion is subjected to a transmission welding to the front material of the file under a state wherein it is caused to abut against the front material of the file.

CONSTITUTION: Back cover part 15 of the front material 14 of a file made of polypropylene resin is put on a surface plate 11. A cover 18 and back cover 19 are formed bendably freely on the right and lift of the back cover part 15 through hinges 16, 17. A binding tool 20 made of polypropylene resin which is of the same quality as the cover material 14 of the file is put on the back cover 15. The binding tool 20 possesses a binding piece 21 and binding pipe 24 which are made bendable freely by a fixed piece 22 and hinge part 23 and the protrusion 26 is provided continuously or discontinuously on a fitting surface side to the cover material 14 of the file. Then an ultrasonic welding horn 12 is lowered, pressurizing force is reduced while pressing the surface 27 of the binding tool 20 by the tip surface 13, the protrusion 26 is allowed to melt by an ultra sonic vibration and welded.