Several Upper House electoral districts are set to be merged after the Diet passed reform legislation Tuesday to rectify sharp gaps in the weight of votes among those constituencies.

In the first merger of Upper House electoral districts since World War II, the revised Public Offices Election Law will take effect before the next election in summer 2016.

The final bill, which came after several drafts and terse negotiations between parties, was submitted by the ruling Liberal Democratic Party and four opposition parties. It was passed by the Upper House last week before Tuesday's approval by the Lower House.