Japan's imperial household law allows only a man to ascend the Chrysanthemum Throne, but no boy has been born to the Japanese royal family since Prince Akishino in 1965.
Emperor Akihito's eldest son, Crown Prince Naruhito, 44, and his wife Crown Princess Masako, 41, have a daughter - Crown Princess Aiko who is three.
Naruhito's younger and only brother Prince Akishino, 39, and his wife Princess Kiko, 38, have two daughters.
If no boys are born in the future to royal women, the Japanese imperial family will theoretically become extinct.
To avert a possible heir crisis, the government will consider adopting a male from former royal members who lost imperial titles after the second world war, the conservative Sankei Shimbun newspaper said on Sunday, quoting imperial sources.
To allow adoption, the government must revise an imperial law, it said, adding some royal family members supported the idea.
Succession law reviewed
A government panel of experts, tasked with reviewing the royal succession, will hold its first meeting on Tuesday.
The panel is set to submit a report to Prime Minister Junichiro Koizumi later this year.
A government panel could revise
the current succession law
The Sankei said the panel could possibly favour the idea of revising the current male-only succession law to allow a woman to ascend the throne.
Since the birth of Princess Aiko in December 2001, there have been mounting public calls for the male-only succession law to be changed.
The latest poll by Japan's top-selling paper Yomiuri Shimbun showed 80% of Japanese supported a female heir.
Royal watchers believe the pressure to produce a male heir has put heavy stress on Princess Masako, who has suffered from stress-induced illness.
The princess, a Harvard graduate who gave up a promising diplomatic career for the marriage, was absent from official duties for about 13 months due to stress.
On her 41st birthday on 9 December, Masako promised she would try to get back to her official duties soon.