A fuel tank flange section molding device including: upper and lower molds that are closed together to mold a fuel tank; a fixed section that controls an amount of molten resin flowing toward a fuel tank main body side due to the protruding portion being disposed nearer to the lower mold side than the mold face when the upper and lower molds have been closed together; a flange insert that includes a mold face capable of rotating about the fixed section to an angle sloped toward a retracting direction of the upper mold, with respect to an extension direction of the flange section; and a cut-off insert that includes a mold face that forms an end portion of the flange section at the opposite side from the fuel tank main body, and that is capable of retracting from the upper mold with respect to the lower mold.