He graduated from the 18th class of the Imperial Japanese Army Academy, where his classmates included Tomoyuki Yamashita and Korechika Anami.

In 1918, with the rank of captain, he was sent to the Vladivostok Special Operations Office during the Japanese intervention in Siberia, and as a major in 1922, he was sent as a military attache to Poland.

During his time in Poland, he acquired the latest code encryption technology from one of his contacts in the Polish General Staff.

After his return to Japan, Okabe served as an instructor at the Staff College from December 1928 to April 1930, during which time he was promoted to colonel.

He was subsequently transferred to command the Mongolia Garrison Army in September 1939, where his forces suffered a defeat against the Chinese during the Battle of Wuyuan.