PURPOSE: To assemble in a mechanically accurate and excellent external appearance by applying a horn to a free end of an upstanding stud provided at a thermoplastic member in the case of clamping two thermoplastic members or thermoplastic member and non-thermoplastic member with one another, and intermittently supplying an ultrasonic energy.

CONSTITUTION: A first thermoplastic workpiece 12 provided with an upstanding stud 14 is supported by a base 10, and a second workpiece 16 made of a thermoplastic material or a non-thermoplastic material is disposed on the workpiece 12. The workpiece 16 has a circular hole 18 through which the stud 14 is passed, which is chamfered 20 to form a recess. Then, a horn 22 is brought into contact with a free end of the stud 14. When it becomes a resonant state in an urged stage, the thermoplastic material of the stud 14 is softened due to dissipation of an ultrasonic energy, and fed into the recess formed via the chamber 20. At this time, a time 30 in which the horn is set to a resonant (operating) state and a time 32 in which the horn is non-operated while being brought into contact with the stud 14 are alternately continued. Thus, the softened and displaced thermoplastic material is solidified to obtain a completed assembly.