In September 1917, Nishi entered Hiroshima Army Cadet School, a military preparatory school established on Prussian models, and in 1920 took courses at Tokyo Central Cadet Academy. One of his classmates was Masanobu Tsuji, formerly head student of the Nagoya Academy. He completed his studies at Central Cadet School in six months in April 1920, due to the reorganization of the military schools, and began courses at the Imperial Japanese Army Academy. Midway through his studies, he was attached to the First Cavalry Regiment based in Setagaya, Tokyo. In 1924, he graduated from the Academy, the 13th of the 19 students in his class, and was commissioned a second lieutenant in October. He went on to the First Cavalry after graduating from Army Cavalry School. He was promoted to lieutenant in October 1927.