Half of the 242 seats in the chamber are being contested.

Polls show Mr Abe's Liberal Democratic Party (LDP) and its allies could secure a majority, meaning a ruling party would control both houses of parliament for the first time in six years.

The deadlock in parliament has been seen as a key factor in Japan's recent "revolving door" of prime ministers.

Polling stations opened at 07:00 (22:00 GMT Saturday) and will close at 20:00 (11:00 GMT).

Despite the importance of the election, early turnout was a little down from the last upper house poll in 2010.

Japan's upper chamber, while not as powerful as its lower house, is able to block legislation introduced by the government.

No single party has a majority, although the opposition Democratic Party of Japan has the highest number of seats.

Opposition parties have had enough combined seats to control the upper chamber in recent years, leading to what has become known as a "twisted parliament".

This has resulted in factionalism and multiple changes of prime minister.

"We need political stability to carry out policies," Mr Abe said ahead of the vote. "We will get that political stability by winning the upper house election."

Mr Abe, 58, has relatively strong public support for his proposals for economic reform known as "Abenomics", which seek to revive the economy, stagnant for two decades.

Since his coalition government came to power, the economy has grown by 4% and the stock market by more than 40%.

His first two measures involved a big injection of cash by the Bank of Japan and a major boost in government spending.

But he now faces the task of driving through difficult structural changes to the economy.

Trade barriers need to come down, taxes will need to rise and large parts of the economy will have to be deregulated, the BBC's Rupert Wingfield-Hayes reports from Tokyo.

One of the decisions he will have to make later this year is whether to raise sales tax next April from 5% to 8% to help reduce Japan's national debt.

Mr Abe is also considering whether to cut Japan's 36% corporate tax to spur growth and open up the power industry, currently controlled by regional monopolies.

And his government is keen to join a free trade agreement known as the Trans-Pacific Partnership (TPP), being negotiated by 11 countries.

'Stable government'

``I want them to carry on doing their best as the economy seems to be picking up,'' one voter, Naohisa Hayashi, 35, told the Associated Press.

"I want to see a stable government. That's the Liberal Democratic Party," 76-year-old Hiroshi Miyamoto was quoted by Reuters news agency as saying, after voting in the western Tokyo suburb of Hachioji.

Mr Abe is also thought likely to endorse several controversial policies beyond the economy.

One is restarting Japan's nuclear reactors - something many in Japan are opposed to.

Another are nationalistic policies that may cause tension with neighbouring countries.

This includes the possible revision of Japan's pacifist constitution, especially a section which prohibits the use of force in international disputes except for self-defence.