The purpose of the present invention is to make it possible to shorten the time required for a burr removal step and make it difficult for the burrs to detach from a sheet member in a step for removing the burrs together with the sheet member. For removal, by making the burrs to adhere to a pliable sheet member such as paper or nonwoven fabric that has been incorporated beforehand in the molding die and stripping the sheet member from the molding die, the burrs are removed together with the sheet member. The present invention is characterized in that in order to make it difficult for the burrs to detach from the sheet member during the stripping, a radiused shape with an arc-shaped cross-section is formed by the molding die on the areas surrounding the sections where the burrs adhere to the sheet member.