We can find many books and articles discussing about public investments or fiscal policy, especially at a time when there are recessions as the Japanese Economy in 1990's. But it seems to be that there have been fewer studies about how the scale of public investments were determined or what was the determinants of them. This paper is devoted to investigating the elements which have determined investments of public sectors, supposing an simple but important assumption; civil servants try to maximize their own utilities which usually are surpluses after required expenditures are paid. Subject to this assumption an investment model of public corporations is made and compared with Model maximizing social welfare. Finally, some interpretations and problems of this analysis are presented.