本文研究本國廠商與外國廠商成本差異與市場結構內生的情況下，本國政府如何制定最適單一進口關稅政策，以提高本國的社會福利。本文發現當外國高成本廠商的成本不會太高時，本國政府為了要留住外國廠商願意課徵一個較低的關稅，但若外國高成本廠商的成本高於某一成程度，本國政府會課徵一個較高的關稅，逼迫外國高成本廠商退出本國市場，對本國社會福利較好。此外，我們進一步探討本國有多家廠商的情況，本文發現當本國廠商家數越多時，本國政府會課徵較低的進口關稅，但在Cournot數量競爭下，外國高成本廠商的生存空間也會縮小，所以會使外國高成本廠商更容易退出本國市場。

This thesis analyzes how the home government determines the single optimal import tariff when the two foreign exporting firms’ costs are different. It is assumed that the home firm(s) and two foreign firms engage in Cournot competition in domestic market. We find that home government should impose a lower tariff in order to retain foreign firm when the cost of high-cost foreign firm is not too high. However, when the cost of the foreign high-cost firm is high enough, the home government should impose a higher tariff to force high-cost foreign firm to withdraw from the home market. In addition, we further find that when there are more home firms in market, the home government should lower the import tariffs. Therefore, it is more likely that the foreign high-cost firm can survive when there exist more local competitors.

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