The introduction of a new organization of common courts (next to the Prosecutor's Office) based on the reception of the Napoleonic Code in the territory of the Duchy of Warsaw was a significant breakthrough in the organization and functioning of the justice system. This complemented the comprehensive reconstruction of the structures of the Polish State in the form of the Duchy of Warsaw, which was desirable from the point of view of socio-political needs and practice. The introduction of modernized administrative courts based on modern French models went in the same direction. Administrative courts as control bodies for acts of public authority marked a significant breakthrough in the functioning of the state. The first part of the study will briefly discuss the method of organizing the central (government) administration of the Principality, which in this form is a novelty in the Polish legal order, bypassing local government and fiscal penal bodies. In the second part, the study will highlight the establishment of administrative courts in Poland.