The construction of particular functions of the Sejm, separated in this paper, results from generalization of conclusions from joining individual competences of the Sejm in homogenous groups. This allows to use presumption of competence within a given function and their further extension. Shaping the system of government takes now the form specified by the constitution. The constitutional function will also include transfer of competence to international organs and organizations. Legislation is an extensive function of the Sejm. Its limits are defined by the legal construction of statute, but also by intensity of legislative practice which in fact determine the role of statute and imposition of restrictions on binding force of internal acts has led to considerable strengthening of the Sejm as a lawgiver. The legislative function is closely related with the function of acceptance of international law. A statute having the attributes of 'formal statute' is a form of acceptance of a large majority of international agreements and recognition of their direct effect and supra-statutory nature. Nevertheless, many international agreements of such character will require statutes enacting transformation of their provisions into norms of domestic law - also in order to enable the government to adopt particular legal acts which cannot be realized in relation to international agreement. Taking the legal construction of statute, designed by the Constitution, as a basis of legislative function permits the separation of a budgetary function. Actions taken within the scope of this function are characterized by particular solutions contained in an annual financial plan (i.e. the State budget). Application of a special procedure to legislative work on the budget should not blur the differences in this act as compared to acts of classically law-giving nature. Making distinction among legislative and budgetary functions leads us to the conclusion that it is inadmissible treat the budget as a legislative act in which provisions amending financial law might be included. The creative and control functions are relatively uncomplicated. Their application should take place with the respect for the principle of non-intervention of the legislative power in the spheres of responsibility of other branches of government.