To seal a functioning material in a resin material by itself by holding functioning materials in parts between a plurality of hot-melting synthetic resin sheets, and welding opposite parts of the sheets holding no materials.

A heat melting synthetic resin sheet 1 is placed between electrodes doubling as press plates 5, 5, and a desired decorative material or functioning material 2 is placed thereon. In addition, the same hot-melting synthetic resin sheet 1 is placed thereon. When high frequency is generated between the electrodes 5, 5, heating begins from the opposite surfaces of the hot-melting synthetic resin sheets 1, 1 and the synthetic resin sheets melt on the surface thereof. Under these conditions, the synthetic resin sheets 1, 1 retain a sufficient shape. Pressing from the upper and lower directions by means of the electrode doubling as press plates 5, 5 allows two synthetic resin sheets 1, 1 to be put together completely in area having no existence of the functioning material 2, and they never separate from each other after cooling.