An acoustic matching device has a water bag (64) that contains water and a water bag support frame (66) that sandwiches and supports the water bag on the left and right sides of the knee and curves along the knee cap. The water bag becomes a plate shape curved along the shape of the water bag support frame, which touches the knee cap and fits the unevenness of the knee. When an ultrasonic probe is moved along the knee cap, the acoustic matching between the knee cap and the ultrasonic probe is obtained.