Prosecutors have questioned industry minister Toshihiro Nikai's brother as a witness on a voluntary basis in connection with an allegation that a company linked to Nishimatsu Construction Co. leased an office for a Nikai-related organization and subsidized the rent, investigative sources said April 7.

The 67-year-old brother, who was operating the organization, has denied the allegation, the sources said.

The prosecutors are investigating the allegation and whether the Political Funds Control Law has been violated, they said.

The Nishimatsu-linked firm has leased the office since around 1999, and provided donations to a political organization related to the economy, trade and industry minister, sources familiar with the case said earlier.

The scandal over Nishimatsu's political donations, in which a secretary of main opposition Democratic Party of Japan leader Ichiro Ozawa was indicted in March, has been spilling over to the ruling Liberal Democratic Party.