The present invention aims to improve the strength of the portion of a cylindrical product, which is made of a fiber-reinforced resin material, in which a weld line is formed. An injection molding die 2 is configured so that by injecting reinforcing fiber-containing molten resin from a gate 13 into a cavity 7 formed between an outer piece 3 and a center pin 4, the reinforcing fiber-containing molten resin merges inside the cavity 7 to form a weld line in the reinforcing fiber-containing molten resin inside the cavity 7. By moving the outer piece 3 with respect to the center pin 4 in said injection molding die 2, the distance to the center pin 4 is changed and the reinforcing fiber-containing molten resin inside the cavity 7 is forced to flow, disrupting the orientation of the reinforcing fibers in the weld line. As a result, the reinforcing fibers in the weld line and near the weld line in the cylindrical product 1 are intertwined, improving the strength of the portion of the cylindrical product 1 in which the weld line is formed.