Notches 23e are filled with part of an elastic material that is injection-molded in a region containing the notches 23e, so that a plate member 23b is taken in by the notches 23e through the repulsive force of the elastic material. Thus, the plate member 23b is secured. Further, the elastic material filling the notches 23e enlarges the joined portion between the plate member 23b and the gasket 24b. Accordingly, the gasket 24b is firmly joined to the surface of the plate member 23b, and can be prevented from being lifted up from the plate member 23b. Thus, the plate member 23b is firmly secured to the separator main body 25.