PURPOSE: To solve the environmental problem due to the destruction of an ozone layer without damaging the appearance quality of a heat insulating box body by using a monomer mixture consisting of a specific amount of a vinyl cyanide compd. and an aromatic vinyl compd. in the presence of conjugated diene synthetic rubber specified in its average particle size.

CONSTITUTION: 84-86 pts.wt. of a monomer mixture consisting of 47-53wt.% of a vinyl cyanide compd. and 47-53wt.% of an aromatic vinyl compd. is polymerized in the presence of 14-16 pts.wt. of conjugated diene synthetic rubber with an average particle size of 0.14-0.22μm in a specific graft ratio within the specific viscosity range of 0.063-0.073 to form a high nitrile thermoplastic resin compsn. composed of a graft copolymer. An inner box 6 is molded from this resin compsn.