Ko Shimomura he tangata whenua no Kōchi Prefecture, a he paetahi o te akomanga 20 o te Imperial Japanese Army Academy i te tau 1908. Ko ona hoa akomanga ko Prince Asaka, Prince Higashikuni Naruhiko, Prince Kitashirakawa Naruhisa, Mitsuru Ushijima, Heitarō Kimura, Takashi Sakai, me Shōjirō Iida. I muri mai ka puta ia mai i te akomanga 28 o te Army Staff College i te tau 1916. I muri i te mahi i roto i te maha o nga kaimahi me nga tuunga whakahaere i roto i te Imperial Japanese Army General Staff, i tukuna a Shimomura ki Parani hei pirihimana hoia i te tau 1919. I hoki mai ia ki te Tari Rautaki me te Whakamaherehere o te General Staff i te tau 1921. I uru ia hei waahanga o te roopu Japanese ki nga whakawhitiwhitinga korero o te Geneva Naval Conference mai i te 1928-1919 me te 1931. Mai i te 1935-1935 ko ia te rangatira o te 1st Army Army Heavy Artillery Regiment, Prince Kitashirakawa Naruhisa, Mitsuru Ushijima, Heitarō
Te tari rautaki me te whakamahere o te General Staff