A shutter piece has a first side surface which seals a peripheral portion of an outer skin material above an inner material, a second side surface which is located below the first side surface and contacts the outer skin material earlier than the first side surface so as to centrally gather the outer skin material toward a location above a center of the inner material, and a third side surface which is located below the first side surface and defines a portion of a pyramid-like space above the inner material when the peripheral portion of the outer skin material is sealed by the first side surface.
