本文利用不完全競爭之本國市場，若存在數家本國與外國廠商，且彼此成本具有差異下，來分析本國政府應如何訂定最適的產業與貿易政策，以提升本國社會福利水準。本國政府最適的產業政策為補貼本國廠商，目的在於矯正市場的扭曲；本國政府最適的貿易政策為對外國廠商課徵關稅，目的在於移轉外國廠商在本國市場所賺取的利潤。此外，我們亦將市場結構此課題納入模型中討論。在本國與外國廠商家數既定的前提下，本文發現，本國政府的最適補貼與關稅政策會受到本國所有廠商之平均邊際成本，以及外國所有廠商之平均邊際成本的影響。最後，市場結構的變動，僅會影響本國政府的最適補貼政策，對於關稅政策則無任何效果。若加入市場結構內生此課題，我們也發現廠商成本差異程度的大小，會對政府的最適補貼與關稅政策產生影響。

This paper considers incompletely competition in home country, which exists several local and foreign firms, and those firms have different cost structure. We analyze how the home government should adopt the optimal industrial and trade policy for improving the social welfare in home country. For home government, subsidizing local firms is the best industrial policy which can justify the market distortion. On the other hand, Imposing tariff on foreign firms, which can transfer the profits from the foreign firms is the best trade policy for home country. Moreover, we also discuss the market structure in the model. If the number of firms in home market is fixed, the average marginal cost for the local firms and the foreign firms will affect on the optimal policies. Lastly, the market structure changes will affect on the optimal subsidy policy but tariff policy. Furthermore, when the market structure is endogenous, the variance of cost structure will affect on the optimal policy as well.

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