The Judicial Affairs Committee of the House of Councilors approved bills Tuesday to cut the monthly salaries of judges and prosecutors by an average of 2.1 percent.
The bills were expected to be passed into law Wednesday with approval of the full Upper House, because they have already cleared the Lower House.
It would be the first pay cut for judges under the Constitution, which came into force in 1947.
The Japanese Communist Party and Social Democratic Party oppose the bills, arguing the Constitution stipulates that judges must receive adequate compensation and it should not be decreased.
However, Supreme Court justices accepted the pay cut in September because, they argued, it would not undermine the independence of the judiciary or endanger the livelihood of judges.