To prevent the poor shielding of a hollow chamber from developing by a method wherein an unavailing foam leaking out to the outer side surface of a holder plate is reduced.

Both the first and the second holder plates 30 and 50 of a holder 20 are openably and closably connected at their one end parts by means of hinge parts 21. Between both the first and the second holder plates 30 and 50, on the inner surface of at least either one holder plate 30, a leg body 32, which passes through a foaming base material 11 and fittingly inserts in the blind hole 51 of the other holder plate 50, is integrally formed nearly normal to the inner surface. Only on one side of the leg body 32, an elastic closing piece 36 having a closing claw elastically engaged with the blind holes 51 is integrally formed. Further, in the holder plate 31, a molding through hole 40 corresponding to the undercut portion of the closing claw is penetratingly provided.