PURPOSE: To obtain moldings having beautiful surface by preventing the occurrence of voids or pinholes by a method in which while a reactive mixture injected is sealed by a specified fluid, the mixture is packed into the cavity of molds.

CONSTITUTION: A reactive mixture, e.g., polyurethane resin, etc., injected from the head 3 of a mixer is sealed by a fluid inert to the components of the reactive mixture and having a boiling point of about -30°C or more, which is packed in a runner. The reactive mixture is then cast into the cavity 6 of molds to obtain a molding having a beautiful surface with lesser defects, e.g., voids, blisters, etc. The fluid used is preferably gaseous ones having a boiling point below the molding temperatures but above about 0°C, e.g., one or more of organic compounds such as halogenated hydrocarbons, etc.