PURPOSE: To obtain a complicate-shaped foamed thermal insulator 'which does not pollute the environment, can be freely injected into a box, and dispenses with the use of a filler for filling the void in the box by incorporating an alternating copolymer component formed from an epoxide compd. and carbon dioxide into the insulator.

CONSTITUTION: A foam is produced by mixing, in a container 1, a polyol, a catalyst such as an amine, water as the blowing agent, a mono- or diepoxy compd. or an oxetane-derived epoxy compd., a zinc, cobalt, aluminum, or tin compd. or an organozinc compd. as the polymn. catalyst, and an isocyanate to therby react water with the isocyanate to form a urethane resin compsn. having urea bonds and simultaneously to generate carbon dioxide. Carbon dioxide generated and the epoxy compd. undergo alternating copolymn. as shown by formula II (wherein R1 to R' are each a substituent) by the action of the polymn. catalyst to form a polycarbonate as the third alternating-polymn. component, thus resinifying carbon dioxide and evacuating the inside of the cells of the foam.