Miwa was a native of Aichi prefecture, and graduated 59th out of 148 cadets in the 39th class of the Imperial Japanese Navy Academy in 1911.

He served his midshipman duty on the training ship Aso and battleship Mikasa, and as a sub-lieutenant on the protected cruiser Chikuma, coastal defense ship Iwami and armored cruiser Azuma.

[3] From 1935 to 1936, Miwa was captain of the light cruiser Kinu,[4] and served as Chief of staff of the Mako Guard District from 1936 to 1938.

[5] His submarines were part of the screening force around Pearl Harbor to prevent any United States Navy vessels from escaping during the air raids.

In July 1944, he became commander-in-chief of the 6th Fleet, in charge of all submarine operations for the Imperial Japanese Navy, and served in this capacity during the Battle of Leyte Gulf in October 1944.