To provide a weld joining technique between a ductile cast iron and a stainless which can reduce the hardness at the weld zone and improve the fatigue strength at the weld zone.

An SUS430 JIL steel pipe 12 having 48.6 mm outer diameter and 1 mm thickness (t) is inserted into a ductile cast iron-made flange 11 having 4 mm thickness (t) near the weld zone and the whole peripheral MIG welding is executed by using a nickel alloy welding material of Y Ni Cr-3 as the welding material 13. Annealing treatment is executed to this welded structural member at 700°C for 1 Hr in a heating furnace.