PURPOSE: To obtain favorable external quality under conventional high temperature coating conditions by a method wherein the temperature of a mold on the back side of a product is raised near or above the glass transition temperature after injection molding under the state that the product is held in the mold so as to heat the back side of the product.

CONSTITUTION: Reaction injection molding is done by mixingly pouring two liquid stocks, mainly consisting of dicyclopentadiene, from an inlet 2. At the above-mentioned reaction injection molding, the temperature of a mold 3 on the surface side of a product is set to be 90°C or while the temperature of a mold 4 on the back side of the product is set to be 70°C or lower. After that, the temperature of the mold 4 on the back side of the product is raised near or above the glass transition temperature for 1 min or longer with an electric heating wire 8 in order to relieve the internal stress on the back side. A split mold 1 is splitted so as to take out a product, from the surface of which dirts are removed by deburring, degreasing and the like. After that, coating is applied to the product so as to dry or harden the coating by baking at a temperature of 89°C or higher under the condition that the product 10 is supported on a coating support 11.