PURPOSE: To reduce notch sensitivity by stress concentration and to improve fatigue strength by providing projecting parts where either one of a side plate or a core plate to be joined to a blade is rounded by the number of blades.

CONSTITUTION: The projecting parts G whose both ends are rounded and an opening hole J where a joint part of the blade 3 is inserted are provided by the number of blades on both corner parts of a Tee joint of the side plate l and the core plate 2 and on the projecting parts G, respectively. The projecting parts G and the opening hole J are chipped off by machining integrally from the side plate 1 and the core plate 2, respectively and with a shape of the opening part J, the width to hold the thickness (t) of the blade 3 from both sides is formed into the depth D corresponding to radius R and the width 2E excepting the opening hole J of the end faces F of the projecting parts G is determined by the welding width, etc., of an electron beam. The joining part of the blade 3 is inserted into the opening hole J and the assembly of the side plate 1 and the core plate 2, and the blade 3 is completed and electron beam welding is performed toward the blade 3 from the outside of the side plate 1 and the core plate 2.