PURPOSE: To prevent the decline of intensity of the sealed section of a pipe material, by providing a welding chip to an ultrasonic horn and, at the same time, forming a curved surface or incline surface in the axial direction of the pipe material on an anvil.

CONSTITUTION: A welding chip 3, to which roller division working is performed, is installed to an ultrasonic horn 2 together with a section 5 for pressurizing pipe material with curved surface provided continuously to the chip 3. A pipe material pressurizing section 7 which is worked to have a curved surface is installed to an anvil 4 continuously to a knurling tool section 6. The knurling tool section 6 of the anvil 4 is positioned below the welding chip 3 and the end section of a pipe material 1 is pressurized with the welding chip 3 and knurling tool section 6. Then the upper plate 1a and lower plate 1b of the pipe material 1 is sealed by using the ultrasonic vibration of the horn 2 intersecting the pressurizing direction at right angles. Since the sealed section is pressurized by the pressurizing section 5 and 7 having curved surfaces, no notched section is produced at the joined boundary. Therefore, decrease of the mechanical strength of the sealed section can be prevented.