PURPOSE: To produce a thermally insulating polyurethane foam contg. few froth voids and excellent in thermal insulating properties with safety by using a blowing agent comprising specific hydrofluorocarbons.

CONSTITUTION: A polyurethane foam is produced by mixing an org. polyisocyanate, a polyol, water, a catalyst, a foam stabilizer, and a blowing agent comprising 20wt.% or less of 1, 1, 1-trifluoroethane and 80wt.% or more of 1, 1, 1, 2-tetrafluoroethane. More specifically, a thermally insulated box is obtd. by mixing the polyol, the catalyst, the stabilizer, and the blowing agent in a specified ratio to give a premix, mixing the premix with the polyisocyanate in a specified ratio with a high-pressure foaming machine, and injecting the resulting mixture into the space between outer and inner boxes to allow the mixture to foam.