This paper explores the family system in the Early-modern Japan Daimyo family taking Okayama Domain of Ikeda as an example. The study shows that Daimyo family is not based on family occupation as was indicated by previous research but rather on consanguinity/blood relation/kinship which is both subject to legal provisions and is also changeable under certain conditions. As a result, succession to house headship is by nature the continuity of official salary in the family, and similarly, the flexibility in the selection of the legal successor, adopted son from a different family or the violation of ethical principles in adopted son, dowry adopted son and youxiliyan(養子離緣) Thus, compared with the influence of pedigree justification and ethics，it would be more greatly affected by reality and situation.