As a lawyer, she was involved in a lawsuit filed by local residents over noise pollution caused by jets taking off and landing at the U.S.

Nevertheless, she strengthened her footing with continuous support by All-Japan Prefectural and Municipal Workers' Union (JICHIRO) and Japanese Trade Union Confederation and took charge of the Director General of a parliament group supporting Amnesty International.

In the course of election canvassing, two executives of the Kawasaki Municipal Transportation Workers Union were arrested because of their involvement in soliciting votes for Chiba for cash payment.

After the 45th general election, she was appointed Minister of Justice in Yukio Hatoyama's cabinet on 16 September 2009.

On 9 October 2009 Chiba granted special permission to two Chinese sisters who lost a lawsuit against a government deportation order.