Nakai was born in Tottori Prefecture. He graduated from the 29th class of the Imperial Japanese Army Academy in May 1917 and from the 42nd class of the Army Staff College in November 1930. As a junior officer, Nakai participated in the Siberian Expedition. In the 1930s, he had extensive experience in China, where he served as a military attaché from 1937. In May 1939, he was promoted to colonel and in September of the same year was assigned to the China Expeditionary Army on the front lines of combat in central China in the Second Sino-Japanese War. From 1939–1940, Nakai served on the senior staff of the IJA 21st Army tasked with the Canton Operation, and from February 1940, on the senior staff of the IJA 22nd Army, a garrison force in Guangxi which fought at the Battle of South Guangxi and which was disbanded for its insubordination in invading French Indochina. Nakai was sent back to Tokyo, but returned later in 1940 as a member of the staff of the Indochina Expeditionary Army.