A method for assembling a metal collar to a resin component includes: inserting the metal collar into a mounting hole of the resin component, and setting them so that a flange of the collar abuts a jig; inserting a screw mandrel of a fastening tool into a through-hole in the collar, and rotating the mandrel so that the male threading of the mandrel engages the female threading of the jig; abutting a nosepiece of the fastening tool against the collar, and stopping the rotation of the mandrel; withdrawing the mandrel so that the nosepiece presses against one end of the collar with sufficient force to enlarge the outer diameter of that end of the collar to form an enlarged portion, thereby securing the resin component between the enlarged portion and the flange; and (e) rotating the mandrel in the reverse direction to disengage the respective threading.