Tanno became a member of the Nankatsu Labor Union, and was especially interested in its communist Hyōgikai wing where she established and was head of its women's division.

[2] She moved when she was 10 years old to Hitachi, Ibaraki prefecture when her father found a job as a carpenter building the Number Two Hydroelectric Plant in Daiyūin, the smelting district.

The spring of her 16th birthday, her family was no longer able to support her financially, so Tanno paid for her clothes and food using the hospital's daily wage.

Tanno was slowly exposed to communist and socialist policy by her male peers, Sōma Ichiro, Kawai Yoshitora, and Kitajima Kichizō, who had picked up socialism from Oka Sensei, a teacher at the nursing college who supported worker's liberation.

[3] Tanno's friend, Kawai Yoshitora, moved to Tokyo in September 1920, but was quickly arrested and sent to prison in Sugamo for his ties to the Socialist Alliance.

In 1921 without fulfilling her required three years of service to the nursing school, she moved to Tokyo's Kameido district with Sōma and Kitajima to reunite with Kawai.

Under Kawai's encouragement, she joined the Gyōminkai (Enlightened People's Society), a communist study group, and began living at their office.

[1][3] During this time, Tanno began studying English at Kanda English-language School and organized a club to analyze Tane Maku Hito, a proletarian magazine.

After an incident where a student of the school attempted to publish poetic works speaking out against the government in a new magazine, Tanno was questioned by police, and her involvement with left-leaning groups was written in newspapers at the time.

[1] In March 1923, she joined the Nankatsu Labor Union on their walk from Tokyo to Noda, Chiba in support of a soy sauce manufacturer's strike.

Upon her return, many of the displaced Nankatsu Labor Union members chose to live in the office, taking turns watching out for the authorities.

[1][6] On September 2, 1923, the Special Higher Police came and arrested between 6 and 8 Nankatsu Labor Union members at their office in an event known as the Kameido Incident.

[1][6][7] Tanno managed to escape by giving her alias Sakanoue Kiyo and running to hide on the second-floor window's ledge behind a sliding tatami door.

The Kameido police continued to make arrests, and many of the union members were killed in prison, including Tanno's nursing school friends Kawai Yoshitora and Kitajima Kichizō.