By the properties that characterize it, Catholic Church is often considered, following the example of the State, as a legal system which takes place inside and outside State. Before the 1905 Act, Gallicanism struggles for independance of the Church of France and the State against papacy, encouraging Sovereign’s interventions in ecclesiastical affairs. Gallicanism can be translated in terms of relations between two legal systems, and the matter of this study is to prove this movement has been a momentum in the intellectual building of Modern State through the reflexion of french jurist from the Ancient Monarchy to the end of the 19th century. Church and State are, by many ways, concurrent systems aiming to dominate the same territory and the same subjects. Gallican thoughts present themselves like an answer to these conflicts. Canonical rules promulgated by the Pope or a Council are not self-executing, and have to be approved by the Sovereign to be Law of the State. Gallicanism build a technical discurse based on State sovereignty to preserve a french particularism against the Roman hegemonic threath. Invented to resolve conflicts of competences between ecclesiastical and State’s authorities, the procedure named « appel comme d’abus » gives exclusive power to the State to determine the extent of its competence. Furthermore this procedure subordinates Church of France to State by the judicial review of its administrative acts.