PURPOSE: To make the manufacture of a resin bumper having guide parts on its inner peripheral surface possible without employing slide cores by a method wherein the portions, each of which opposes to the tip parts of a resin bumper guide part, of the first mold are made to be tapered surfaces expanding to the outward of the first mold.

CONSTITUTION: After a resin bumper A is molded in a space 13 by pouring resin in the space 13 formed by molds 10, 11 and 12, the resin bumper A is removed from the molds. In order to remove the bumper A from the molds, first, the mold 11 is slid off. Secondly, the bumper A is brought into the state so as to be capped at the front of the mold 10 in order to apply forces P1WP3 which push the resin bumper A to the outward of the first mold 10. In this case, because the portions, each of which contacts with the tip part B' of a guide part B provided on the inner peripheral surface of the resin bumper A, of the first mold form tapered surfaces, the resin bumper A can easily be removed along the tapered surfaces 10a from the first mold 10.