In this paper, the relation to the bone union was clarified by measuring the bone in vivo strain with a strain gauge. First of all, the device which measures a load in vivo for a long time with a strain gauge system is developed, and, as a result, it was able to be done that the bone in vivo strain was stably measured for 20 days. Next, result of measuring fracture model by using this system, it was able to be confirmed that it began to be able to do a callus during about the seventh day of after the operation in the fracture part, and dynamic strength of the bone rose rapidly with the progress of the bone union then, too.
