An echo tracking processor unit detects a bone surface section based on an echo signal and tracks a position of the bone surface section. An interpolation line generator unit generates an interpolation line connecting tracking points which represent the tracked bone surface section. A translational displacement corrector unit removes a translational displacement component between an interpolation line corresponding to a time phase of a no-load state stored in a memory and an interpolation line corresponding to a time phase in a loaded state output from the interpolation line generator unit. A strain calculator unit calculates an amount of strain of bone for each time phase and outputs to a characteristic curve generator unit. The characteristic curve generator unit generates a characteristic curve related to the strain of bone based on a load value output from a load measurement device and the amount of strain of bone output from the strain calculator unit.