The present invention solves the problem that a connecting portion between a horn and a vibration transmission rod has a tendency to get damaged and the problem associated with the structure in which a chip is attached to an end of the vibration transmission rod, which occur with the known ultrasonic vibration welder. A vibration transmission rod has an end surface serving as a vibration-and-pressure application surface, and a polygonal cross section. The vibration transmission rod is disposed at an end of a horn in such a manner that they vibrate together. The vibration transmission rod and a mass are formed separately. The mass applies pressure to a projection formed at a nodal point of the vibration transmission rod.