PURPOSE: To enable easy lap welding to the front end of the 1st object to be welded without opening the spacing between a backing strip and a clamper so much by lifting the front end of the 2nd object to be welded by means of a lifting guide via a vertical driving mechanism.

CONSTITUTION: An object 7 to be welded is clamped to a backing strip 1 by a clamper 2, whereafter a clamper 3 is opened by a pressurization mechanism 5 to open the spacing between the strip 1 and the bottom surface of the clamper 3 so far the bottom surface of the clamper 3 becomes higher than the height as much as the sum of the height of an object 8 to be welded and the height at the front end of the object 7. While a lifting guide 11 is held raised to about the same as the height at the front end of the object 7 or the position slightly higher than the same by a vertical driving mechanism 10, the front end of the object 8 is inserted from the side. Then the front end of the object 8 overlaps automatically on the front end of the groove 9 for welding through the top surface of the guide 11. The guide 11 is lowered from the surface of the strip 1 while the front ends are superposed, then the object 8 is clamped onto the strip 1 by the clamper 3 and both objects are welded by a welding torch 6.