If Abe, who resigned abruptly after a troubled 2006-2007 term, stays in office through November 2019, he will have exceeded the 2886 days marked by Taro Katsura in the early 20th century.

"I want to tackle constitutional reform together with all of you," Abe told his Liberal Democratic Party after the vote on Thursday.

First, however, he has the immediate challenge of a likely summit with US President Donald Trump next week, when he will face pressure to cut Japan's $US69 billion ($A95 billion) surplus with its key ally, nearly two-thirds from vehicle exports.

Abe must also keep economic growth on track with a dwindling policy tool-kit.

After years of heavy money printing, the Bank of Japan has little ammunition left. Japan's huge public debt and rising social welfare costs for a fast-ageing population also leave Abe with little room to ramp up fiscal spending.

Abe, who surged back to power in 2012 promising to reboot the economy and strengthen defence, defeated former defence minister Shigeru Ishiba in the LDP leadership election.

Abe won 553 votes to Ishiba's 254. Of the 810 votes up for grabs from LDP parliamentarians and rank-and-file party members, 807 were valid.

Abe is assured the premiership because his LDP-led coalition controls parliament.

The Trump administration is exploring raising tariffs on Japanese vehicle exports, a step Japanese officials say would do serious damage to the two economies and world trade.

At home, the premier's "Abenomics" recipe of hyper-easy monetary policy, spending and structural reform has helped reflate growth, although critics say reforms came up short.

Now he is pledging to reform social security, making it easier for people to stay in the workforce and offset Japan's shrinking population by raising the retirement age to above 65 and letting them defer pension payouts beyond age 70.

Abe is also promising to invest in infrastructure to cope with the natural disasters that increasingly batter Japan, though more spending could make it harder to rein in debt.

He has said he will implement a planned rise in the sales tax to 10 per cent from 8 per cent in October 2019 but that could hit the economy just as Trump's protectionist policies could hurt Japanese exports.

High among Abe's personal priorities is revising the post-war constitution's pacifist Article 9 to clarify the military's ambiguous status. The article, if taken literally, bans maintenance of armed forces but has been interpreted to allow a military for self-defence.

Pushing for it would be politically risky since the public is divided. Amendments require approval by two-thirds of both houses of parliament and a majority in a public referendum.

Australian Associated Press