PURPOSE: To foam a material for a packaging vessel at a using site by forming a resin material layer to be foamed by heating on the surface of a base plate to be formed in the vessel.

CONSTITUTION: For example, foamable beads 3 are adhered in a layer state on the surface of a plate sheet (base plate) 1 with adhesive 2. When it is used as a vessel, it is cut in a vessel developed plate A2 to be assembled, formed in a vessel A3, heated, a vessel V4 in which it exterior is formed of the plate sheet 1 and the interior is formed of a foamable resin layer 3 is formed, and used for packaging. Accordingly, since it is thin and not bulky before the vessel is assembled, it is optimum as a packaging box for frozen fish, etc. for an ocean fishing ship having a limited cargo space. Since the foamable resin layer 3 is adhered upon foaming, the formation of the vessel A3 may be temporarily clamped.