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The purpose of this study is to develop a new ultrasound diagnostic system for non-invasive and quantitative assessment of mechanical properties of the bone or bone healing. In the previous papers, we reported that we had developed a new ultrasound system to measure a minute bone deformation using a multi-point echo-tracking (ET) and that it had a great potential for non-invasive and quantitative diagnosis of bone healing. In this paper, we present a newly developed measurement system with improved accuracy for assessing deformation of intact tibia in vivo. It consists of a dedicated probe, a transmitting/receiving system and analysis software calculating a minute bending angle of the bone surface under a three-point bending (TPB) test. And, we report results of a performance evaluation of the developed system by using test measurements. Furthermore, we evaluated the reproducibility of the in vivo measurement by repeatedly measuring the bending angle of the tibias of 5 healthy volunteers every week for one month. As a result, the evaluation of the accuracy of the measured bending angle using the metallic plate for calibration showed that the standard deviation (SD) of the measurement in range of 0 to 0.1 degrees was 0.004 degrees. Then, we performed an in vivo measurement of normal tibia. The results showed that the mean bending angle of the normal adult tibias under a load of 25 N and a supporting span of the tibial length of each subject was 0.058 degrees with a SD of 0.01 degrees. In addition, SD of the data for the measurement repeatability was 0.006 degrees. We developed a bending angle measurement system for the human tibia using a TPB test and obtained an excellent accuracy of the system and also confirmed through the measurement of the tibia of human volunteers that the repeatability was sufficient to quantitatively assess bending property of the intact tibia.