Prince Hitachi, the younger brother of former Emperor Akihito and uncle of Emperor Naruhito, was hospitalized with a fever Sunday, the Imperial Household Agency said.
The 87-year-old was recovering well after surgery to crush stones in his ureter in March, but he was readmitted to the Japanese Red Cross Medical Center as a precaution, the agency said.
After running a fever of over 38 C Saturday evening, he took an antipyretic drug, bringing his body temperature to below 38 C, but his appetite was not as good as usual, it said.
Prince Hitachi is third in line to the throne after Prince Hisahito, the 16-year-old son of Crown Prince Fumihito.
Prince Hitachi, Emperor Naruhito's uncle, discharged from hospital
Prince Hitachi, Emperor Naruhito's uncle, undergoes surgery
Prince Hitachi, Emperor Naruhito's uncle, hospitalized for fever