The paper makes an attempt to identify the essence of the principle of ex officio investigation, which is considered common to administrative procedure and administrative proceedings. The authors set the task of analyzing not only the similar manifestations of this principle in the procedure and proceedings. An equally important for understanding the essence of the principle, and therefore the correct application of procedural and processual norms, is, according to the authors, to distinguish its different manifestations that cannot be ignored, because this principle forms the basis for the activities of representatives of various branches of power, namely executive and judicial. In order to perform the stated task, the paper analyzes among the rules of the Law of Ukraine "On Administrative Procedure" and the Code of Administrative Proceedings of Ukraine those that characterize the considered principle, it is demonstrated in comparison how the representatives of each branch of power use them. As a result, the paper states that the representatives of executive and judicial authorities, under the influence of the principle of ex officio investigation, demonstrate evident activity in proceedings: they are authorized to take actions on their own, without obtaining the consent of the interested parties, which guarantee that the decision made by them in the case will fully meet the requirements declared at the regulatory level. At the same time, such activity is ensured by norms formulated taking into account the nature of the activities of each of the subjects of power, and therefore, in its manifestations, it cannot be recognized as identical.