A Lower House panel approved a bill on Friday to improve the treatment of criminal suspects in police custody awaiting trial or sentencing.
The ruling coalition, made up of the Liberal Democratic Party and New Komeito, voted for the bill while opposition parties, Democratic Party of Japan and Social Democratic Party, voted against it.
The bill contains a provision maintaining the controversial police detention facilities known in Japanese as "daiyo kangoku," or substitute prisons, which are used to hold detainees prior to indictment.
The daiyo kangoku system has drawn criticism at home and abroad for increasing the chances of human rights abuse.
Meanwhile, the Judicial Affairs Committee of the House of Representatives voted down a revision bill that contains a provision that gradually decreases the number of detainees in daiyo kangoku. The bill was jointly proposed by the DPJ and SDP.
The approved bill is expected to be adopted by the Lower House's plenary meeting Tuesday.
The government will submit the bill to the ongoing Diet session and aims to enact it during the current session due to end June 18.
The bill calls for clearly separating police officers in charge of investigations from those who supervise detainees to ensure they are treated fairly.
In Japan, the treatment of pretrial or presentencing detainees is currently governed by the prison law, which was established about 100 years ago.