A mounting unit (16) is aligned by using a mounting unit frame (58) touching the thigh and a knee-contacting portion (62) touching the knee, with the knee being bent. The mounting unit is also equipped with a probe support frame (60) used for moving an ultrasonic probe along the knee cap. The probe support frame moves rotationally about the mechanical center of the knee joint, thereby allowing the ultrasonic probe (78) to move along the knee cap while facing toward the mechanical center of the knee joint and to perform scanning of an ultrasonic beam.