Utilization of superconducting permanent magnets is now under investigation at SPring-8 for future development of synchrotron radiation sources, and several concepts has been reported so far. In one of them, ring-shaped bulk superconductors (ring SCs) will be used for enhancement of the undulator field strength. In the previous work, it was reported that the mechanical property of the ring SCs were so poor that the magnetic performance degraded during experiments. In this work, several methods are proposed to enhance the mechanical property of the ring SCs and results of experiments performed to investigate the effects of them are reported. Resin impregnation with a pole piece inserted into the ring SCs is found to be effective.