An upper casing is outer-fitted to a lower casing. A concave portion for receiving a peripheral portion of a housing is provided at an inner wall of the upper casing. A convex portion for suppressing deformation of the housing caused by deformation when the upper casing is shrinkage-fitted or pressed-fitted is selectively formed at the bottom of the concave portion. A gap is provided between the lower casing and the housing for alleviating the degree of press on the housing by the casing when the upper casing and the lower casing are welded.