The commanders Yasuyuki MATSUI and Tatsuyuki ARIYOSHI at Kitsuki-jo Castle asked Josui for reinforcements and, in response, Josui moved his army and clashed with the army of Yoshimune OTOMO at Ishigakibaru (presently Beppu City) on October 19.
城将・松井康之と有吉立行は如水に援軍を要請、如水はこれに応じ9月13日(旧暦)（10月19日）、石垣原（現在の別府市）で大友義統軍と衝突した。