After Yoshimoto's prominent assistants successively died of illness, such as his strategist Sessai dying in 1555 and Yasuyoshi ASAHINA in 1557, the Imagawa family gradually began to weaken after living at the height of its prosperity.
弘治(日本)元年（1555年）に軍師・雪斎、弘治3年（1557年）には朝比奈泰能といった名補佐役が次々と病死し、次第に今川家の全盛期に翳りが見え始める。