A distal side transmitting member of an ultrasonic transmitting unit includes a distal side abutment portion located at a midway position different from antinode positions and node positions, and a projecting portion provided to protrude from the distal side abutment portion toward a proximal direction. A proximal side transmitting member includes a proximal side abutment portion abutting on the distal side abutment portion, and a depressed portion provided to be depressed from the proximal side abutment portion toward the proximal direction and engaging with the projecting portion. A gap dimension between a protruding end of the projecting portion and a bottom portion is set so that the ultrasonic transmitting unit vibrates at a predetermined resonance frequency.
