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We built a Kerr microscope to observe magnetic domains in Nd-Fe-B sintered magnets at elevated temperatures of up to 100°C. The microscope was equipped with a heating stage and set in an electromagnet that applied a large magnetic field. During observations, a variable dc magnetic field of up to 14 kOe could be applied, and the temperature could be controlled up to 320°C. A 5-nm-thick Ta layer was found to prevent oxidation during domain observation at elevated temperatures of up to 320°C. We observed nucleation and growth of the reversed domains beyond grain boundaries with increasing temperature. This indicates that decrease in the coercivity of the magnet at elevated temperatures is attributable to decrease in anisotropy and insufficient pinning of domain walls at grain boundaries.