The ageing of the population and the imbalance of public finances force governments to carry out pension reforms in order to insure the sustainability of pension systems. The reforming of social security systems is becoming even more urgent as the government ability to cover the deficit of pension funds with transfers from federal budgets is limited. The paper presents the modification of overlapping generations model developed by [Bettendorf, Heijdra, 2006]. The model was extended to account for the unbalanced pension system and endogenous interest rate, important in estimation of pension expenses. We consider an optimal combination of fiscal instruments depending on the retirement age, life expectancy and productivity of labour and compare social welfare in the case of balanced and unbalanced pension system. The welfare analysis shows that financing the pension fund deficit via income taxes is a part of optimal policy. It was also shown that when the deficit of the pension fund is covered by the government, income tax and social contributions are perfect substitutes when the interior solution is considered. Thus in case of balanced pension system optimal social contributions are positive and are used to finance pensions, while optimal income tax rate does not depend on the rate of population growth. In the case of unbalanced pension system, the maximization of welfare function points in favour of corner solution with zero social contributions and positive income tax, which depends on population growth, retirement age and labour productivity. While an unbal an­ced pension system with optimally chosen fiscal instruments allows to achieve higher social welfare due to higher level of capital per efficiency unit of labor and lower equilibrium level of public debt.