A medical instrument is disclosed for crushing a thrombus present in a pulmonary artery of a living body by an ultrasonic vibration. The medical instrument includes an elongated main body section capable of being inserted into an airway of the living body, and an ultrasonic oscillation section which can be inserted into the living body by the main body section and which oscillates the ultrasonic vibration inside the living body. By the medical instrument, a thrombus present in a pulmonary artery can be rapidly crushed, whereby a therapeutic effect on pulmonary embolism can be obtained.