The President’s oath of office is a solemn declaration taken in the presence of the National Assembly by which a new head of state swears to be faithful to the provisions of the Constitution; to steadfastly safeguard the dignity of the Nation, the independence and security of the State, and also that the good of the Homeland and the prosperity of its citizens shall forever remain his supreme obligation. The oath of office has been taken by all Presidents of the Republic of Poland, from Gabriel Narutowicz in 1922 to Bronisław Komorowski in 2010. Over the span of 90 years the words and the form of the oath of office were subject to changes. In the interwar period, the President’s oath of office was religious in nature and was articulated in following words „I swear to Almighty God, One in the Holy Trinity”. After World War II, i.e. between 1947 and 1952, the President’s oath of office partially lost its sacral dimension, although it was still ended with obligatory Invocatio Dei. Nowadays, namely from 1992, secular oath of office may also be optionally taken with the additional sentence „So help me, God”. President’s oath of office is an example of a specific type of legal institution that originated from interaction between different normative systems. This institution combines law with morality, religion and custom. Legal relevancy of the President’s oath of office reveals itself in the fact that swearing-in is a condition which President has to fulfill to take over the office. Whereas infringement of the oath of office can be one of the grounds, rather than the only ground, for Presidential impeachment before The Tribunal of State. Swearing-in is a crucial element of Presidential inauguration. Official schedule of the first day of the Presidential term also contains taking control of the Armed Forces and receiving insignia of the two highest polish orders (The Order of the White Eagle and The Order of Polonia Restituta). The President also participates in the Eucharist held in the Archcathedral in Warsaw, which is rather a private ceremony. Characteristic of the given institution requires not only the analysis of the law but also its practice. Analysis of statues and other normative acts should be accompanied by empirical case study.