Composite materials such as sandwich panels have been employed in many structural applications. Therefore, it is very important to obtain the dynamical properties of the composite materials. Also, an inverse analysis method has already been proposed by one of the authors to identify the layer elastic parameters of laminated composite materials using the FEM eigenvalue analysis and the sensitivity analysis. The purpose of this study is to improve the proposed identification method to apply to a sandwich panel and to identify the equivalent elastic parameters of a core material. By applying the experimental modal analysis technique to the sandwich panel, natural frequencies and mode shapes of the panel are obtained. From the obtained natural frequencies and mode shapes, the equivalent elastic parameters of the core material are identified numerically. Furthermore, the validity of the identified equivalent elastic parameters was shown.