Although the Matsui clan was one of retainers of the Hosokawa family, they were regarded as a domain lord almost equal to daimyo, since they practically ruled vast land of 30.000 koku of Yatsushiro Domain in Higo Province.
松井氏は細川家臣でありながら、実質上の肥後八代支藩3万石の大名格藩主であった。