A female screw portion 24 is formed over the entire length of a through hole 23 passing through an axis of an ultrasonic horn 13, first and second male screw portions 25, 26 formed on the ultrasonic horn 13 side of axes of first and second boosters 14, 15 are screwed into the female screw portion 24, the ultrasonic horn 13 and the first and second boosters 14, 15 are fastened coaxially, leading ends 27, 28 of the first and second male screw portions 25, 26 are not in contact with each other inside the through hole 23, and a cutting blade 12 is attached to an outer periphery of a base portion 20 of the ultrasonic horn 13 having a space portion 29 surrounded by an inner wall of the through hole 23 and the leading ends 27, 28 of the first and second male screw portions 25, 26.