At J-PARC Main Ring (MR), there have been various investigation carried out at the moment aiming at the beam operation of MW order. As one of the investigations, a study of the Rapid-Cycling Synchrotron (RCS) magnets was implemented. Increase of the extraction energy of RCS was needed to reduce beam loss, as beam loss in the MR injection region was large under influence of the space charge effect at the 3GeV 1MW beam from RCS. Therefore design study of the extraction energy upgrade using dipole and quadrupole magnets of RCS was performed. In this paper we will report the contents of the study for upgrading the extraction energy of RCS magnets and problems found out as a result.