An outer housing 12 formed of a sheet metal is provided such that its open end portion 312 surrounds an outer edge portion 26 of an inner housing 11 formed by casting. An outer protrusion 216, an annular protrusion 227, and a connecting portion 27 are provided on the outer edge portion 26 of the inner housing 11. The open end portion 312 of the outer housing 12 is joined to a protruding end 217 of the outer protrusion 216, a protruding end 228 of the annular protrusion 227, and an end edge 271 of the connecting portion 27, whereby the inner housing 11 and the outer housing 12 are integrated.