TOKYO — Prime Minister Shinzo Abe has said that he wants to start the process of revising Japan’s Constitution as early as next year, a senior lawmaker in his party said Thursday, giving the clearest indication yet that the Japanese leader will seek to change a document that has undergirded the country’s postwar pacifism.

Mr. Abe told Hajime Funada, the leader of a group of Liberal Democratic Party lawmakers, on Wednesday that the best time to begin the difficult political task of amending the Constitution would be after elections for the upper house of Parliament, scheduled for the summer of 2016, Mr. Funada said. Mr. Funada’s comments, which were originally reported by two Japanese newspapers, The Asahi Shimbun and The Sankei Shimbun, were later confirmed by his political secretary, Miki Mori.

When Mr. Funada asked the prime minister whether the best time for starting the revision process would be after the upper house elections, Mr. Abe answered, “That would be common sense,” according to Mr. Mori.

According to Mr. Mori, Mr. Abe did not specifically say that he wanted to rewrite Article 9 of the Constitution, which prohibits the use of armed force in resolving international disputes. But this week, Mr. Abe told Parliament that he wanted to make that change in order to strengthen Japan’s hand in responding to threats like the Islamic State militant group, which killed two Japanese hostages last month.