私人費用在所得稅法上的意義介於天秤的兩端，一邊作為可支配的所得運用，並作為課稅的基礎；另一邊則為不可支配的所得運用，並在課稅禁區的範圍內。後者，德國稅捐實務以憲法生存權、家庭以及婚姻的保障為基礎，認為滿足稅捐義務人及其家庭成員最低生存必要的費用必須從所得稅的稅基中扣除。然該立論基礎以及重要性並未普遍獲得我國稅捐實務之承認，相反的，不斷藉由立法者的形成自由或是稅捐機關的行政行為，徹底侵犯稅捐義務人的稅捐自由，此透過歷年相關大法官解釋以及我國所得稅法免稅額、扣除額的規定可稽。甫施行的納稅者權利保護法第4條，在其施行細則第3條的解釋適用下亦無法達成其原制定之立法目的。因此，本文在此背景下展開，並藉由德國法的發展，探討基於憲法的價值而在所得稅法上必須作為「不可避免的」費用項目以及與此相關的立法者的類型化權能以及裁量自由的界限。

The meaning of private costs in the income tax law is between the ends of the scales, as the use of disposable income and as the basis for taxation; the other side is the use of unencumbered income, and as a restricted area of taxation. In this context, the German tax law from the Constitution of the right to life, that meet the tax obligor and his family members to the minimum subsistence necessary costs must be deducted from the income tax base. However, the basis of the Constitutional Protection and the specific design of the system of income tax exemption and other deductions are not universally recognized by Taiwan's tax practice. On the contrary, the legislators use the formation of freedom, complete violate taxpayer which has the right not to be taxed. Therefore, this research project explores what is meant by the constitutional value the ＂unavoidable＂ costs in the income tax law, and the limits of authority and discretion space of legislators.