A plate body includes: an inner bulge protruded on one side of the panel body and recessed on the other side and has a periphery spaced from attachment parts; connector bulges having a certain width protruded on the one side of the plate body and recessed on the other side and connect the periphery of the inner bulge to the attachment parts; and an outer bulge recessed on the one side of the plate body and protruded on the other side and continuous with the inner bulge, the connector bulges and the attachment parts. The inner bulge, the connector bulges and the outer bulge of the plate body are expansion-molded and include a skin layer formed on the surface and has high resin density and an expanded layer formed therein and has a plurality of voids and low resin density relative to the skin layer.
