PURPOSE: To improve processing accuracy and the durability of an apparatus in textile sealing, etc., by pressing an ultrasonic energy transmission horn to a workpiece for a specified period and changing the power transmitted from the horn to the workpiece by a control signal during this time.

CONSTITUTION: This resonance horn 19 to be pressed to the workpiece is connected to an electro-acoustic transducer 18 in the case, for example, the welding of the rigid thermoplastic workpiece is carried out. This electro-acoustic transducer 18 is connected by a conductor 17 to a wattmeter 20. The wattmeter 20 is connected to by a conductor 16 to a power source 8. As a result, the electric power is supplied from the power source 8 to the electro-acoustic transducer 18 and the ultrasonic energy is supplied from the horn 19 to the workpiece. The supply electric power of this time is detected by the wattmeter 20 and the signal corresponding to the power applied to the transducer 18 is outputted to a comparator and a latch circuit 21. When this input signal exceeds the variable reference signal inputted from an adjustment controller 22, a signal is outputted to an electric power control circuit 500 to control the power source 8, by which the moving amplitude of the horn 19 is changed.