Dewa was born as the son of a samurai of the Aizu domain (present day Fukushima prefecture).

He served as a junior officer on several vessels of the early Japanese Navy, including the corvette Tsukuba, ironclad warship Fusō, sloop Hōshō, ironclad warship Ryūjō, corvette Amagi, and cruisers Asama, Naniwa and Takachiho.

[2] During the First Sino-Japanese War (1894–95), Dewa served as a staff officer of the "Western Seas Fleet", a defensive force patrolling home waters.

Promoted to rear admiral on 20 May 1900, and vice admiral on 6 June 1904, during the Russo-Japanese War he was a commander of the 3rd Cruiser Squadron of the IJN 1st Fleet and took part in the naval Battle of Port Arthur, and the Battle of the Yellow Sea, (where he commanded from the cruiser Yakumo).

In December 1905, he was appointed Commander-in-Chief of the IJN 3rd Fleet, and from November 1906 was Director of the Naval Education Bureau.