立憲政友会
In 1911, Shimada opened a legal office and began work as a lawyer. He was elected to the Lower House of the Diet of Japan in the 1912 General Election as an independent, but joined the "Rikken Seiyūkai" the following year. He was re-elected nine times, and rose within the "Rikken Seiyūkai" to eventually become Secretary-General of the party from 1937 to 1939.