module this attachment portion 98 is provided at a 90 degree interval in the circumferential direction of the flange portion 19 and it has attachment holes 99 for receiving screws an outer edge of the flange portion 20 on a closure shell side is indicated by the dotted line in fig2 the closure shell 2 is formed by pressing a stainless steel sheet it has a circular portion 30 a central opening 15 formed at its center a circumferential wall portion 47 formed on the outer circumferential portion of above mentioned circular portion 30 and a flange portion 20 which is located at the tip of this circumferential wall portion 47 and which extends outward in the radial direction a bent portion 14 bent in the axial direction is provided at an edge of the central opening 15 the bent portion 14 provides rigidity to the edge of central opening 15 and provides a relatively large joint face with the inner cylindrical member 16 the inner cylindrical member 16 is provided so as to fit into the central opening 15 and an end surface 17 of one end of the inner cylindrical member 16 is designed to become level with the end surface 18 of the bent area 14 the diffuser shell 1 and the closure shell 2 form the housing 3 by overlaying the flange portion 19 of the diffuser shell with the flange portion 20 of the closure shell at the central cross section in the horizontal direction with respect to the axial direction of the housing 3 these shells are connected by laser welding 21 the flange portions 19 and 20 provide rigidity to the housing especially to its outer circumferential wall 8 to prevent deformation of the housing due.