Nagayasu was the son of Miyoshi Yoshikata and was adopted by Sogo Kazumasa, inheriting his Sanuki province domain. He was defeated at the Battle of Nakatomigawa by Chosokabe Motochika and was later forced to submit. He readily joined the Toyotomi when Hideyoshi ordered the invasion of Shikoku in 1584. He also joined in the expedition to help the Ôtomo defend their capital of Funai against the Shimazu in 1587. He took part in the Battle of Hetsugigawa and was killed in the course of the fighting.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005