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.
