The Swedish legislation on Custody, Residence and Access enacted in 1998 declares that “the best interests of the child” should be the primary consideration in any decision. In custody litigation, each parent makes claims alleged to be in the “best interests of the child”. The purpose of this paper is to analyze the rhetoric and structure of the debate carried out in order to justify their claims, and to clarify how the concept of the “best interests of the child” is construed in court decisions, using the Toulmin model through legal cases in district courts in 2004 to 2005.　In custody disputes, mothers plead “domestic violence by fathers” or “serious conflict between the partners” as facts, and claim sole custody for mothers for the reason that the father is incompetent as a caregiver, or that joint custody is difficult due to lack of communication. Fathers deny these facts, and claim joint custody using the rhetoric of “the need for fathers”. In residence disputes, fathers plead as a fact “children remaining in their home is good”, and claim residence should be with fathers, using the rhetoric of “the wishes of the child” and “rearing experience”. Mothers plead that staying in the father’s home is bad, and respond with the rhetoric of “manipulation of the child’s wishes by the father”, “different sense of nurturing”, “problems with his father’s upbringing”, and claim residence should be with the mother. In visitation disputes, against the father’s complaint of “visitation interference by the mother”, mothers argue with the rhetoric of “denial of visitation by their children”. Against the mothers’ complaints of “risks of visitation”, such as sexual abuse, violence, or kidnapping by fathers, fathers deny these risks and argue with the rhetoric of “the importance of interaction with the father”.　The academic knowledge of “what is best for the child” after the separation of parents is fluid. Treatment of joint custody has swung like a pendulum during this decade. Court decisions on “what is best for the child” have changed.