Method and device of diagnosing bone strength

A bone strength diagnostic device includes a wave-transmission module that transmits an ultrasonic wave obliquely to a bone covered with soft tissues; a wave-reception module that receives the ultrasonic wave that exits from the bone to the side of the soft tissues; a shape detection module for detecting the shape of the front surface of the bone; and a speed-of-sound deriving module for deriving the speed of sound of the ultrasonic wave that propagates along the front surface of the bone, based on the received wave signals, and the shape of the front surface of the bone detected using the shape detection module.