PURPOSE: To stabilize heater current and enhance reliability of welding by forming a low melting point metal to reduce contact resistance in a contact section between a rod electrode and a heater.

CONSTITUTION: A low melting point metal to be melted by electric current and reduce contact resistance between a rod electrode 4 and a heater 1 is filled in a contact section 5 between the electrode 4 and the heater 1. That is, when the current is applied to the heater 1 for welding, for example, solder is filled in the contact section 5 between the electrode 4 for applying the current and the heater 1, and when the current is applied, said solder is so melted as to vary the contact section 5 from point contact to surface contact and reduce contact resistance. The applied current is stabilized to the operation for a long period of time by said arrangement, and reliability at the time of plastic welding can be enhanced.