According to US reports (available from the CIA’s FOIA website) Onodera was born in September 1897 in Iwata, Japan. He came from a prominent family and pursued a military career. In the period 1912-1920 he studied first at a local cadet school, then at the central cadet school in Tokyo and finally at the Military Academy (infantry course). During his time at the academy he learned Russian.
In order to continue his intelligence activities on the Soviet Union he was appointed military attaché to Latvia in 1936, where he established close ties with the military and intelligence authorities of that country and the other Baltic nations of Estonia and Lithuania. These countries were fearful of the Soviet Union and they were willing to exchange secret intelligence with Japan.
After a brief stint back to the General Staff in Tokyo in 1938 and then an assignment in the Expeditionary Force in China he was given the important position of military attaché in the Japanese embassy in Stockholm, Sweden. He officially occupied this position from February 1941 till the end of the war.
This post used to have limited value in the field of intelligence but during the war the Soviet occupation of the Baltic countries and the German occupation of Poland meant that many intelligence officials of these countries found refuge in neutral Sweden. Both the Baltics and Poland had ties to the Japanese intelligence service, especially in their anti-Soviet activities. Finding themselves without a country many of Onodera’s former acquaintances had no alternative but to make a living by selling secret intelligence.
Another country with anti Soviet policy was Finland. In the 1930’s relations with Finland were strengthened in the intelligence field. The Japanese were impressed by the performance of the Finnish codebreakers, especially during the Winter War.
Onodera’s contact with the Polish IS was his close associate Michal Rybikowski, a former intelligence officer, whose cover name was ‘Piotr Ivanov’. Onodera protected him from the Germans and the effort paid off as he received valuable information on the German military, the Soviet Union and the fighting in the Eastern front.
Was Onodera’s support of Rybikowski completely justified? A US report says that though him the Poles and the British were able to monitor the Japanese activities.
Considering the close relationship between the Japanese and the Polish IS it is possible that Rybikowski betrayed only parts of Onodera’s activities to the British.