To stabilize welding quality by performing the control of welding on the basis of the heating temp. of a welding iron and the displacement quantity thereof up to the target processing treatment after the melting of a part to be welded.

The moving quantity of a welding iron 4 controlled to predetermined heating temp. from an initial position to a position where the leading end of the welding iron is brought into contact with an article to be welded to melt the same by predetermined quantity is detected by a displacement quantity detection means 12. A drive control part 8 for controlling the rising and falling movement of the welding iron compares the detection output of the displacement quantity detection means 12 with a preset value and controls the lift mechanism 3 of the welding iron 4 at a time when the moving quantity of the welding iron reaches the set value to raise the welding iron 4.