PURPOSE: To cure a heat-curable prepolymer in a short time without adversely affecting plastics and metals by mixing the heat-curable prepolymer with a curing agent and bringing the resulting mixture into contact with hot vapor of a specific perfluorinated organic compound.

CONSTITUTION: A heat-curable prepolymer is mixed with a curing agent and this mixture is brought into contact with hot vapor of a perfluorinated organic compound, provided that the compounds represented by formulae I and II are excluded, which hot vapor has a temperature necessary for curing at least the prepolymer to be used. In the formulae m, n are each 0-3; and p is 1-9. As the perfluorinated organic compound to be used, 5-20C linear chain or branched perfluoroalkanes, e.g. a perfluoroalkane C8F18 having a boiling point of 102°C, perfluoro-2,3,4-tri-methylpentane C8F18 having a boiling point of 115°C, etc., are illustrated.