A heat insulating body comprising a rigid polyurethane foam of open cell structure, and a vessel formed of metal-plastics laminate film enveloping said rigid polyurethane foam, the whole system being evacuated to reduce the internal pressure and then hermetically sealed; and an adiabatic box comprising said heat insulating body, an inner box and an outer box, said heat insulating body being disposed between said inner box and said outer box and the whole system being subjected to integral foaming with rigid polyurethane foam.