The method manufactures a vehicular seat member including a frame member and a foamed resin molded body to embed the frame member. Firstly the frame member is placed in a molding die so that the frame member extends along the outer shape of the vehicular seat member in plan view of the vehicular seat member. Next the foamed resin molded body is formed in the molding die so that the foamed resin molded body has a rim part embedding the frame member and having an outer shape of the vehicular seat member and an extension extending inward from the rim part so as to bridge the rim part inside of the rim part. During the forming, the method forms a break in the extension so as to divide the extension along the direction intersecting the extending direction of the extension.