In an interview with London's Financial Times newspaper, Gen Nakatani, Defence Agency director-general, hinted that he would like the military forces to have a more prominent status in Japan and to play a greater role in overseas peace-keeping missions.
But in his first interview with the foreign press since his appointment in April, Mr Nakatani said it was time to have another look at it.
"The Japanese constitution has remained the same for the 46 years since it was enacted," he told the newspaper. "At that time, Japan was under occupation ... the Japanese understand and approve of the idea that the Self-Defence Forces are playing a central role in the defence of Japan."
"I believe we need to have thorough discussions among ourselves in order to make it clear that Japan will not wage a war of aggression in the future, but that Japan still needs to defend itself, and the Self-Defence Forces will play that role," the general said.
Tricky diplomacy
The comments are likely to be welcomed in Washington, which has long hoped for Japan to take a more active role in regional security. Mr Nakatani is due to fly to the US later this month ahead of talks between President George W Bush and Japanese Prime Minister Junichiro Koizumi.
But Mr Nakatani's comments could also add to existing tensions with China and South Korea. Both countries have criticised Mr Koizumi's plan to visit the Yasukuni Shrine to Japan's war dead in August. South Korea is also angry at a new Japanese textbook which it says glosses over Japan's wartime atrocities.
Any talk of expanding the military could also cause political problems for Mr Koizumi, who has already spoken of the idea. New Komeito, the second largest party in his ruling coalition, has warned it may reconsider the alliance if Mr Koizumi's Liberal Democratic party moves too far towards the right.
"We support Mr Koizumi on his reform ideas about the economy but we are very unhappy about the nationalism, and think it could affect relations [in Asia]," said Otohiko Endo, chairman of New Komeito's international affairs committee.
Coalition rift?
Meanwhile Mr Koizumi has not ruled out opposing US plans for a missile defence shield - a departure from the government's position of complete neutrality.
"We have to carefully consider this issue, which has enormous influence on global security," he told parliament on Wednesday.
His comments came amid an ongoing controversy over Foreign Minister Makiko Tanaka's reported comments that she had doubts about the US plan.
Her reported remarks, allegedly made to foreign ministers from Italy and Australia, caused an uproar in Japan because they clash with Tokyo's official stance that it "understands" Washington's plans.