According to the Department of Cultural Affairs of Japan (2017), there are 239,597 Japanese language learners currently in Japanese language schools and universities in Japan, less than 10% of the total number of foreign residents in Japan. Japanese language education has mainly focused on foreign students and children while overlooked immigrants who have come to Japan for work or marriage. It has been reported that some of them are experiencing difficulties in learning Japanese due to lack of opportunities to attend Japanese language classes and little contact with Japanese people and the Japanese community. This study interviewed a Nepalese migrant who come to Japan for his family. Based on the sociocultural approach by Bonnie Norton(2003), the present study focused on the "power" structure that surrounds him to clarify various aspects of his second language acquisition process. It also focused on the process of change in the "agency" and aims to describe how his agency changes as he learns Japanese. The study found that the Nepalese migrant is placed in a subordinate position within a power relationship such as "native Japanese speakers/non-native Japanese speakers" and "Japanese/non-Japanese", and that he avoided using Japanese because of feeling ashamed and fear of speaking out in such power relations and was unable to learn Japanese. On the other hand, the Nepalese immigrant is motivated to learn Japanese by negative experiences such as discrimination and prejudice, and he began to take the initiative to counter such power relations after they started learning Japanese in evening classes at the public junior high school attended by immigrants with similar backgrounds. The results revealed that there is a link between learning Japanese and building agency that can resist power.