New Party Sakigake
He served as vice prime minister and finance minister from 5 January to 7 November 1996 in the first cabinet of Prime Minister Ryutaro Hashimoto that was a coalition of the Liberal Democratic Party, the SDP and New Party Sakigake. Kubo's term ended when Hashimoto inaugurated his second cabinet and the coalition parties SPD and New Party Sakigake remained outside the government. Kubo was succeeded by Hiroshi Mitsuzuka as finance minister.