PURPOSE: To form penetration beads balanced on both sides in the fillet parts of a perpendicular plate on a rear surface in Tee joint welding of narrow parts of welded impellers of compressors, etc.

CONSTITUTION: The through-energies LPE, RPE of a welding heat source Q penetrated through parts to be welded by welding these parts are separated with the perpendicular plate a in-between and are respectively measured by detectors LS, RS. The two measured values of the through-energies are compared and the position of the welding heat source is automatically controlled to be maintained always adequately by using a position controller K in such a manner that both balance. An adequate weld metal is formed in the fillet parts of the tee joint, by which the strength and above all fatigue strength of this joint are improved.