An ultrasonic probe includes a probe main body section to which the ultrasonic vibration is transmitted from its proximal end to its distal end; a curving section which is bent relative to the probe main body section; and a treating section which is continuous with a distal portion of the curving section to treat the knee joint. The curving section has a central axis which is bent at an angle of 5 degrees or more and 8 degrees or less relative to the longitudinal axis of the probe main body section, and is always disposed in a projection plane of the probe main body section, when the probe main body section is seen along a longitudinal axis from the proximal end toward the distal end.