The subject of the article is to identify factors and conditions that determined the system of government of the IInd Republic of Poland under the Small Constitution of 1919. This act served as a temporary constitution until the March Constitution of 1921 came into force, which happened completely only at the end of 1922. Under the Small Constitution there has been made an attempt to introduce the system of supremacy of the parliament. It turned out to be impossible because of high authority of the head of state – Józef Piłsudski, who also served as the Commander-in-Chief. Therefore, the system of balance between the Legislative Sejm and the Chief of State was shaped in the political practice.