A shutter piece (3, 3', 103, 203) has a first side surface (13, 113, 213) which seals a peripheral portion (DE) of an outer skin material (D) above an inner material (F), a second side surface (15, 115, 215) which is located below the first side surface (13, 113, 213) and contacts the outer skin material (D) earlier than the first side surface (13, 113, 213) so as to centrally gather the outer skin material (D) toward a location above a center of the inner material (F), and a third side surface (17, 117. 217) which is located below the first side surface (13, 113, 213) and defines a portion of a pyramid-like space (17a, 117a, 217s) above the inner material (F) when the peripheral portion (DE) of the outer skin material (D) is sealed by the first side surface (13, 113, 213).
