In August 1925, he became commander of the Tenth Cavalry Regiment (at the rank of captain) and graduated from the 38th class of Army Staff College.

Prince and Princess Kaya undertook a seven-month world tour in 1934, visiting the United States, Great Britain, France, and Germany.

After his return to Japan, he replaced his uncle, Prince Asaka Yasuhiko, as the emperor's personal envoy to Nanking, the occupied capital of Nationalist China, following the Nanjing Massacre in January 1938.

Prince Kaya became commander of the Third Imperial Guard Division in 1944 and briefly served as president of the Army Staff College during the closing stages of World War II.

Barred from holding public office because of his military career, the former prince received a lump payment from the reconstituted Imperial Household Council in order to "maintain his dignity."