After serving as captain of the Hashidate, he was appointed assistant captain of the Asama on 23 August 1914. This was the day that Japan declared war against the German Empire, and Yoshioka went to war in World War I. Initially, he was part of the 1st Southward Expeditionary Force led by Yamaya Tanin to search for the German fleet led by Maximilian von Spee, and engaged in the occupation of the South Seas Mandate.[1] Next, he was part of the US-led branch led by Keizaburo Moriyama to protect trade from South America to North America. However, 31 January 1915 off Mexico, the ship struck an uncharted rock at the entrance to the bay.[2]
In December 1915 (1915), he became captain of the Tsukuba. She was commissioned by the Taishō Emperor at a fleet review held the following year.[3] After serving as captain of the Kongō, he was promoted to Rear Admiral in 1917 and became the second director of the Naval Education Headquarters. On 1 December 1919, he became Chief of Staff of the First Fleet, the following year he became Chief of Staff of the Combined Fleet, in 1921 he was promoted to Vice Admiral and Director of the Naval Gunnery School and in 1923, served as a member of the Naval General Staff Council. He became a reserve officer in 1924.
As a vice admiral, he was awarded the Order of the Golden Kite, 4th class. He was also known as the "God of Gunnery". He died in 1930. His commandment name was Seichuin-den Atsuo Tanchu Daishi and his grave is in Ogawa, Uki, Kumamoto Prefecture.
