Japanese media reports say the prince had been hospitalized since May, initially because of pneumonia. Citing unnamed sources, they say his condition took a sudden turn for the worse on Thursday.
The death of the younger brother of the late Emperor Hirohito coincides with renewed attention to the future of the ageing and shrinking imperial family and whether women should be allowed to inherit the throne, breaking a males-only succession tradition that stretches back 2,600 years.
Born December 2, 1915, Prince Mikasa had been fifth in line to the throne. His brother Hirohito reigned for more than 60 years, during World War II and until his death in 1989.
A scholar of ancient Oriental history, he taught at colleges, and served as honorary president of the Middle Eastern Culture Center in Japan and the Japan-Turkey Society.
The current regent upon the Chrysanthemum Throne, 82-year-old Emperor Akihito, hinted in August that he wanted to abdicate - a step unprecedented in modern Japan and not possible under current law. The remaining four male heirs include 10-year-old Prince Hisahito, the emperor's only grandson.
The three older heirs are Akihito's 80-year-old younger brother and his two middle-aged sons including Crown Prince Naruhito.
jar/kl (Reuters, AP)