The identification of external force acting on a machine or a structure is important for diagnosis. One of the force identification methods is based on the frequency response function (FRF). The method is very useful although it requires the information of the location of acting force. In this study, an identification method is proposed for the case of unknown excitation points. In the method, the inverse problem of force identification is regularized by increase the number of FRF by adding a mass on the target structure. The validity of the method was checked by the numerical example and the experiment for a beam structure elasticity supported at both ends. To solve inverse program, two regularization filters, which are the truncated singular value decomposition (TSVD) filter and Tikhonov filter, are used in the numerical example. It was shown that the accurate identification results can be obtained by TSVD filter. But in the experiment, the accurate results cannot be obtained by using TSVD filter.