The purpose of the current invention is to provide a bone strength diagnosing device and a bone strength diagnosing method capable of accurately diagnosing a bone strength of a bone. A longitudinal ultrasonic transducer and a transverse ultrasonic transducer radiate ultrasonic waves onto a bone front surface of a tibia from a predetermined angle to generate surface waves that propagate in the bone front surface in a longitudinal direction of the tibia and in a direction perpendicular to the longitudinal direction, respectively, and receive leaky waves leaking to the side of the soft tissues from the front surface waves. A speed-of-sound calculating module calculates speeds of sound of the surface waves in the transverse direction and the longitudinal direction based on a difference between course lengths of the ultrasonic waves and a difference between propagation times. A first index calculating module calculates a first index by which a bone density is evaluated using the speed of sound of the surface wave in the transverse direction. A second index calculating module calculates a second index by which an orientation of the bone is evaluated using the speeds of sound of the surface waves in the longitudinal direction and the transverse direction. A third index calculating module measures a cortical bone thickness and then calculates a third index by which a bone strength is evaluated. A bone diagnosing module uses at least one of the first index, second index, and third index to diagnose the bone strength.
