PURPOSE: To contrive to make manufacture efficient, by a method wherein molding is performed by making use of a filler neck molding core to which a microscopic protrusion having a flat surface at a position against which either a pipe molding pin or the pipe molding pin of a molding die is abutted against is provided.

CONSTITUTION: A filler neck molding core C is provided with a microscopic protrusion 3 having a flat surface 3a which is vertical to a position against which a pipe molding pin B of a molding die Aa is abutted against. As resin molding is performed under a state wherein the pipe molding pin B of the molding die Aa having a vertical and flat surface is abutted against the microscopic protrusion 3 having the vertical and flat surface 3a of the filler neck molding core C, a resin tank T is molded without generating resin burrs within a pipe 2. In other words, penetration of resin is prevented by adhering favorably and closely the pipe molding pin B of the molding die Aa having the flat surface to the vertical and flat surface 3a of the microscopic protrusion 3 by abutting them against each other. With this construction,when either the pipe molding pin or the pipe molding pin of the molding die is made to abut against the microscopic protrusion, both of them adheres favorably and closely, and a gap is eliminated, through which manufacture efficiency can be contrived all the more.