In the neighborhood area X of the tongue part 27 forming the winding end part of the scroll part 3, the wall part members 31 are provided on both the sides of the welding part (the facing part) ‘a’ of the first scroll part 5 and the second scroll part 7, wherein: the wall part members 31 encloses the welding part (the facing part) ‘a’ and forms the gas-tight space 33; and, the outer circumference wall which enclose the facing part and forms a gas-tight space configure a double-wall structure.