The 3-GeV Rapid-Cycling Synchrotron (RCS) at the Japan Proton Accelerator Research Complex (J-PARC) has achieved the designed high power beam operation at 1 MW output. The study of an upgrade plan is then started to realize the 1.5-MW beam power. Regarding the radiation protection at the upgrade plan, a new injection system has been proposed to secure enough space for radiation shielding and maintenance work. For this purpose, it is necessary to integrate the splitted iron cores of the injection shift bump magnet into one core, the length of which is shorter than the total length of the splitted cores. The number of coil turns for the new one core magnet is then increased from 2 to 4. The structural design of the new shift bump magnet excited at 25Hz repetition rate is in progress from view point of eddy current losses at the magnet edge and the coil temperature by using the OPERA-3D. This paper details these aspects and outlines the new power supply briefly.