A distal side transmitting member of an ultrasonic transmitting unit includes a projecting portion protruding from a distal side abutment portion toward a proximal direction, and a proximal side transmitting member includes a depressed portion depressed from a proximal side abutment portion toward the proximal direction. A cavity portion having a space is formed between a protruding end of the projecting portion and a bottom portion of the depressed portion inside the proximal side transmitting member when the depressed portion is engaged with the projecting portion, and one of antinode positions of an ultrasonic vibration is located within a range from the protruding end to the bottom portion.