A bone assessment apparatus for diagnosing bone by transmitting and receiving measuring waves (in particular ultrasonic waves) is provided. A body part (e.g. a foot) is gripped by a pair of transducer assemblies, and ultrasonic waves are transmitted and received in this state. Each transducer assembly comprises an ultrasonic transducer and a coupler. The transverse cross-sectional area of the ultrasonic wave beam is adjusted by adjusting the contact area of the coupler on the body part, by changing the vibrating area of the ultrasonic transducer, or by fitting an attachment which narrows the ultrasonic wave beam to the transducer assembly. The transverse cross-sectional area of the ultrasonic wave beam may be changed depending on the size of the body part to be measured.