It is crucial to grasp wheel-rail contact forces in the evaluation of running safety and curving performance of railway vehicles. To measure the wheel-rail contact forces, instrumented wheelset, which has the strain gages on the wheel surface, is widely used. The purpose of this research is to increase the measurement accuracy of the wheel-rail contact forces by understanding the detailed characteristics of the instrumented wheelset. Although the various research works on the instrumented wheelset have been carried out to increase the measurement accuracy of wheel-rail contact forces, there are few works considering the longitudinal force and the lateral shift of the wheel-rail contact point. However, sometimes the longitudinal force has a non-negligible influence on the measurement accuracy on the instrumented wheelset. In this paper, the authors clarify the cross-sensitivity characteristics of the instrumented wheelset when the longitudinal force is applied to the various lateral position on the wheel tread through the finite element method (FEM) analysis and the static load test. The authors also propose a method to approximate the cross-sensitivity as an analytical function of the lateral and circumferential contact positions.