A graduate of Waseda University, he was elected to the House of Councillors for the first time in 2007 after serving in the assembly of Shiga Prefecture.

[2] On December 15, 2010, he attended the United Nations Security Council Ministerial Meeting on Iraq in New York as a representative of the Japanese government.

Three United Nations Security Council resolution were adopted at the meeting, and Tokunaga has also made a speech on the SDF's contribution to Iraq's reconstruction.

[7] In the 2017 Japanese general election, he ran as a Kibō no Tō candidate from Shiga 4th district, but lost to Hiroo Kotera of the Liberal Democratic Party, who ran on behalf of Mutō, who retired due to a money scandal over stocks.

[11][12] In the 2021 Constitutional Democratic Party of Japan leadership election (held on November 30) following the resignation of Yukio Edano, he endorsed Kenta Izumi.