The House of Representatives decided Wednesday to extend the current 150-day Diet session by 42 days to July 31 to give the ruling bloc more time to pass key bills.
Lawmakers from the ruling Liberal Democratic Party, New Komeito and the New Conservative Party approved the extension by a majority vote.
The Democratic Party of Japan voted against the proposal, while the other three opposition parties — the Liberal Party, the Japanese Communist Party and the Social Democratic Party — boycotted the vote.
The session had been scheduled to end Wednesday.
The move marks the first extension of an ordinary Diet session since 1999.
Prime Minister Junichiro Koizumi, who also heads the LDP, pursued the extension to conduct further debate on key bills.
These include medical reform measures and a package of bills related to the deregulation of postal services. Other major bills concern emergency attack-response measures and protection of personal information.
The ruling coalition filed the 42-day extension with both Diet chambers Tuesday.
Under Diet law, extending a session does not necessarily require a vote in the Upper House, although the chamber planned to discuss the matter at a board meeting of its steering committee on Wednesday.
The Diet has been idle for more than a week since opposition parties boycotted procedures to protest alleged interference by the ruling bloc in the release of a Defense Agency report relating to a privacy-violation scandal involving agency officials.
The ruling bloc is apparently seeking to get Diet debate back in motion, but the opposition parties want both the coalition and the government to reveal who tried to smother the report on June 11.
During a news conference that day, the agency released just a summary of the report, which focuses on the compilation of lists of people who had contacted the agency seeking information, as well as personal information about them.
It released the full report later that day amid angry protests from the opposition camp.
During the extended Diet session, the government and the ruling parties plan to postpone debate on the emergency measures bills and those related to privacy protection, seeking instead to prioritize legislation aimed at revising the health insurance law and deregulating postal services.
The Lower House was expected to pass a bill to amend the health insurance law on Thursday.
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