The currency unification in European countries started in January 1999. This ED's trial to introduce the common currency "Euro" is expected to promote the productive efficiency and stimulate economic activity in the area, but it also includes several issues to maintain and expand this new currency system. Which currency system is suitable for European countries, flexible exchange rate system, fixed exchange rate system, or common currency system? What conditions are necessary to adopt the common currency "Euro"? This paper summarizes the benefits and problems of this currency unification from the standpoints of "Optimum currency area theory" with real economic indicators. Then this paper argues the currency issues in terms of financial and fiscal aspects. Finally, this paper suggests the important role of Euro currency in the international currency system.