To provide a knee joint rotation angle measuring instrument using ultrasonic waves capable of easily and accurately measuring the rotation angle of the knee joint in a relatively simple constitution.

This knee joint rotation angle measuring instrument consists of a thigh locking means, a lower thigh rotating means capable of rotating the lower thigh region about its longitudinal axis, a femur image acquisition means for irradiating ultrasonic waves to the femoral region and acquiring the image of the femur surface part, a tibia image acquisition means for irradiating the ultrasonic waves to the tibia region and acquiring the image of the tibia surface part, and an image analysis means, wherein the image analysis means has a rotation angle calculation means for calculating the rotation angle of the tibia to the femur based on a first femur image and tibia image acquired before rotating the leg region and a second femur image and tibia image acquired after rotating the leg region.