The Lower House passed a bill Tuesday that would give adults under guardianship the right to vote.
The bill to amend the public offices election law was approved unanimously sent to the Upper House.
That chamber is expected to pass the bill Friday. The Upper House election this summer will likely be the first campaign where adult wards will have the right to cast a ballot.
The bill would remove a provision that adults placed under guardianship lose their voting rights. To prevent any wrongdoing, only those engaged in clerical work at polling stations would be able to cast ballots on behalf of the wards, according to the bill.
With the revision, some 136,000 adults under guardianship will be granted the right to vote.
After a woman under guardianship sued the government, the Tokyo District Court ruled in March that the election law clause banning wards from voting in elections is unconstitutional.
Internal affairs minister Yoshitaka Shindo said that once the bill is enacted, the plaintiff in the lawsuit would no longer see the need to sue the government and the trial would be brought to an end.
However, the government will not withdraw its appeal, he said.