本論文主要在討論外資所得稅的問題。現今世界中有些國家為了保證其國內產業（尤 其是幼稚產業）因此對其進口財課征關稅。然而這樣不僅使消費者須付高價而消費。 并且造成生產面的資源配置扭曲，而使國內福利水準降低。 在面對此種兩難的政策下，政府只好退而求其次今另行其他次任的政策以補救國內福 利水準降低的情形；其中－－政策亦即是本文所討論的主題：對外資所得的課稅，如 此可便資源扭曲的情形較為改善。 本論文第二節將設立一基本模型，第三節將討論外資所得稅稅率之決定，而第四節為 結論。 事實上，國際貿易理論中最佳貿易的情形品國際自由的貿易下受任何政策的干擾；因 此若在自由貿易的條件下 適外資所得稅稅率似乎應為零。然而在面對關稅的干擾下 ，最適稅率不在保持中文，而應隨著關稅而有所變化，以明使國內的福利損失情形不 致達過惡化。以上便是論文主要探討的大致內容。

有鑑於歐洲各國的福利政策與福利支出相較於其他國家高，歐洲國家多會提供窮困與殘障的人民最基本的社會保障，一般人民亦可享受到基礎醫療保障，在失業時也可以領取失業補助；歐洲各政府甚至會收購即將倒閉的企業或者提供補助使企業能夠繼續經營。然而在持續延燒的歐債危機下，為維持歐元區普遍的薪資和福利水平，歐元區內國家開始採取了國家借貸的做法，但由於國際經濟形勢不佳，歐元區經濟增量未達預期，許多國家原有債券陸續到期，若無法借到新貸款，國家將面臨倒閉危機。因此本論文主要以動態隨機一般均衡模型(DSGE)，探討一個封閉經濟體系下，政府支出進入家計單位效用函數後，換言之政府支出變動將會影響家計單位消費的邊際效用，試著尋找能夠極大化社會福利的政府公債占國內生產毛額的最適比例。我們發現在提高政府公債占國內生產毛額比例時：家計單位將預期未來稅賦繳納之金額更高，因而減少消費、增加儲蓄，所以會排擠掉部分私人消費；同時政府必須提撥部分費用以支付債券利息，所以將排擠掉部分政府支出。另外隨政府公債占國內生產毛額比率上升，整體社會福利水準會逐漸下降；因此，我們認為最適政府公債占國內生產毛額比率應為零。 關鍵字：動態隨機一般均衡模型、政府支出、邊際效用、公債、國內生產毛額、社會福利水準 / In Euro zone, large spending obligations needed to support the welfare state and redistribute wealth in an effort to gain greater equality. Most European countries provide liberal social security benefits to the poor, disabled, basic medical needs and very liberal unemployment benefits. They also own and run large public companies. Under the ongoing Euro debt crisis, European governments figure out the way to maintain the high welfare level by increasing the public debt they hold. In our paper, we would like to investigate the optimal ratio of public debt to GDP by constructing a micro-based dynamic stochastic general equilibrium model (DSGE) under a closed economy. We focus on discussing the optimal ratio of public debt to GDP which maximizes social welfare in economy while putting the government expenditure into households’ utility. That is to say, the government expenditure will influence the marginal utilities of households. We find out that as a government decides to raise the debt to GDP ratio; it will crowd out both private consumption and government expenditure. Because households will expect to pay more tax in the future, they will decrease their consumption and increase their saving; high debt ratio means government should have paid more interest payment in the future. Also as the ratio of public debt to GDP rises, the social welfare becomes lower. Thus, in our findings, the optimal debt ratio to GDP should be 0. Keywords: DSGE, Government expenditure, Marginal utility, Public debt, GDP, Social welfare