PURPOSE: To uniformalize the welding quality by maximizing the AC power impressed on an electrode at the medium and by minimizing it at the start point and end point in a parallel gap seam welding.

CONSTITUTION: The cylindrical bottomed container 23 made of ceramics is prepared and a semiconductor element 25 is mounted on the bottom part thereof, then a weld link 20 is fixed with brazing to the opening end of the container 23 and a conductive cover body 21 is deposited with a parallel gap seam welding method as well. In this case, the AC welding power which welds the seam major side and minor side is divided at equal intervals so as to maximize the power thereof at the medium, and yet it is impressed so that it becomes mini mum at the start point and end point respectively. In this way, an uniform welding is made possible even in case of the pass resistance being varied with the moveing of the weld position. Consequently, the welding quality can be uniformized.