The purpose of this study was to investigate the excitation force that generates the vibration of the reduction gear case for railroad vehicles. This excitation force is difficult to measure directly. The inverse stiffness method was used using the acceleration response measured in the experiment and the vibration response function derived from the finite element analysis. It was assumed that the excitation force acting on the reduction gear operates in the X, Y, and Z directions for each bearing, and a total of 12 excitation forces were investigated. When deriving the excitation force, singular value decomposition was applied to the vibration response function to increase the accuracy of the result. The results of the excitation force according to the number of degrees of freedom of the response were compared. Additionally, the magnitude of the estimated excitation force according to the singular value category used was compared, and it was confirmed that a too low singular value indicates a different excitation force.