To prevent an abnormal heating from occurring, even when a short- circuiting is caused by first detecting the initial value of an electric current for fusion bonding when the electric current is applied to an electrofusion joint at a specified voltage.

When a fusion bonding process is initiated by pressing a start button, a switching means 1 is turned on by a control signal from CPU 6 and an electric current for fusion bonding at a constant voltage is applied to an electrofusion joint 2 from a constant voltage circuit. In turn, CPU 6 calculates a resistance based on an electric current value measured by an electric current measuring means 4 and stores a set time corresponding to the resistance in RAM 8 after obtaining the time from ROM 7. In addition, CPU 6, referring to a clock generator 9, stores an electric current value when a reference time reaches the level of time when the voltage is stable in RAM 8, if the reference time arrives at such a level. Thus CPU 6 controls an electric current adjusting means 3 so that the electric current value stored in RAM 8 is maintained as it is until an energization time set thereafter elapses. The termination of the energization is performed by the control signal from CPU 6 when the reference time reaches the set time.