Reliability of bolt/nut assemblies directly influences the quality of machines and structures. It is most important to control and maintain the clamp force of bolt/nut assemblies to secure reliability. However it is not easy to control the clamp force during tightening. And it is also difficult to detect the clamp force after tightening. In our previous study, we proposed a method to easily detect the clamp force of bolt/nut assemblies that have already been tightened. In the method, the bolt thread portion protruding from the nut is pulled while supporting the nut’s upper surface. The relationship between the tensile force and the displacement at the pulling point differs before and after the tensile force reaches the clamp force. The method can detect the tensile force at the point where the relationship changes as the clamp force. In this study, robustness of the detection method for the squareness error of the bearing surface of clamped parts has been investigated by FE analysis and experiments. The results of the FE analysis showed that the squareness error of the bolt bearing surface does not have an influence on the detection accuracy. The results indicate that the detection method has robustness for the squareness error of bolt bearing surfaces. Contrary to that, the experimental results show that the squareness error does have a clear influence on the detection accuracy. The averages of detection error in the experiments increased about 10% with an increase in squareness error. It is considered that the error was caused because the mating thread surfaces did not separate when the tensile force reached the clamp force.