A front engagement face 27f of a front engagement member 27 and a rear engagement face 29f of a rear engagement member 29 are respectively configured to be located a little back from a virtual plane VF coplanar with one side of a platform 15 and also to be substantially parallel to the axial direction of a male dovetail 25, and an end face of a front wall Wf integrally molded in the vicinity of abase portion of a front seal fin 21 and an end face of a rear wall Wr integrally molded in the vicinity of a base portion of a rear seal fin 23 are respectively configured to be coplanar with the virtual plane Vf.