To eliminate a joint failure to be generated at making a horn of an ultrasonic machine abut on parts of a writing utensil from top face, when the parts are to be welded by an ultrasonic process in the joining method.

When the parts of a writing utensil are to be welded and joined together by an ultrasonic process in the joining method, the horn of the ultrasonic machine is brought into contact with the parts from the side of the writing utensil. Consequently, it is possible to firmly fix the parts and prevent characters on the top face of a tail plug or like from being collapsed.