TOKYO—Japan is starting legislative procedures to legalize gambling, opening parliamentary discussions on a bill to promote resort casinos.

Masahiko Shibayama, chair of the lower house's cabinet committee, said the bill would be explained at a committee meeting Wednesday. Passage of the measure is likely to be postponed until at least autumn.

The bill, which is being submitted by Hiroyuki Hosoda, the deputy secretary-general of the ruling Liberal Democratic Party, would be the first part of a two-stage process that could eventually lift a ban on gambling in the world's third-largest economy. Upon passage of the bill, the government would be asked to take legislative steps within a year to open up the market for the gambling industry.

For proponents who have been pushing the issue for more than a decade, Wednesday's move is a important step forward. Talk of legalizing casinos has been floated in Japan for many years, but more important bills and frequent administration changes have kept the issue on the periphery.

"We have to remind ourselves that this is just the start, and until we see gambling become legal, we must keep up our efforts," said Toru Mihara, an adviser to politicians pushing the legislation.

Japan could become one of largest gambling markets in the world. Brokerage CLSA, for example, has pegged the country's potential revenue from gambling at $40 billion, more than six times the Las Vegas Strip's $6.5 billion total last year.

With such a large expected impact on the long-stagnant economy, Prime Minister Shinzo Abe is counting on casino resorts to attract travelers from overseas. The nation hopes to raise the number of visitors to 20 million a year by 2020, from 10 million last year.

"Integrated resorts will be a main feature of my growth strategy," said Mr. Abe when he visited casinos in Singapore last month.

Still, developments related to how the bill is being handled suggest that everything isn't full steam ahead. Advocates were originally hoping the bill would be passed during the current parliament session, which will end on June 22. But swamped by the large number of bills the committee had to handle, the schedule kept being pushed further into the future.

Now politicians say passage won't take place during the current 150-day session.

The failure disappointed industry advocates, who say it drastically slowed down the momentum for legalization.

"As an ex-Wall Street guy, it's all about deal momentum," said MGM Resorts International MGM -0.08% MGM Resorts International U.S.: NYSE ^$24.79 -0.02 -0.08% June 17, 2014 4:02 pm Volume (Delayed 15m) : 3.40M AFTER HOURS ^$24.79 0.00 0.00% June 17, 2014 4:26 pm Volume (Delayed 15m): 9,478 P/E Ratio N/A Market Cap $12.17 Billion Dividend Yield N/A Rev. per Employee $163,499 06/17/14 In Japan, Casino Push Moves Ah... 06/13/14 MGM Gets Approval to Build Mas... 06/13/14 Japan Ready to Bet on Casino G... More quote details and news » MGM in Your Value Your Change Short position Chief Executive Jim Murren in May.

"When you lose the deal momentum, sometimes you lose the deal," added Mr. Murren, who is based in Las Vegas.

But lawmakers dispute such a view. Koichi Hagiuda, a senior LDP lawmaker, said casino operators won't be welcomed if they quickly lose interest just because passage in the current session is unsuccessful.

After discussions, advocating lawmakers plan to vote on the bill, send it to the upper house and pass it there—all during the Japanese parliament's autumn session. The committee is scheduled to have one more meeting during the current session on Friday, but steps to wrap up the current session leave no room for the bill to be discussed that day.

Still, the plan might not proceed as scheduled, even though Wednesday's kickoff is expected to give the bill priority over some others during the coming session. Some lawmakers say passing the measure during the autumn session won't be easy, especially if any important bills get in the way, because the extraordinary autumn sessions don't usually last long.

Lawmakers opposed to the bill, such as the Japan Communist Party's Mikishi Daimon, have been making efforts to prevent the bill from being discussed because its large support base among lawmakers makes passage likely once debate begins.

Mr. Daimon, together with Japan's bar association, warns of negative repercussions from bringing casinos to Japan, including crime and corruption.

At a recent news conference, Chief Cabinet Secretary Yoshihide Suga stressed the benefits casinos could have for the economy, saying that the government will be sure to set up crime-control systems.

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