本論文首先探討何謂公法上之不當得利，先以概述之方式開啟本論文對此議題之探討，又所謂民法上之不當得利，乃源自衡平法上的概念，係為調整雙方之間的關係；而公法上不當得利則不在於強調雙方之間的衡平，而係依法行政，其該給多少，該拿多少，接係基於依法行政而來，該制度非在針對國家公權力行為所造成財產上損害之賠償，而係因「財產變動」、「基於公法關係而生」以及「無法律上原因」三者而發生，可見係純以欠缺法律上原因所造成之財產上變動，請求回復其財產狀況。公法上不當得利制度，非在於填補因國家違法或合法行為所致財產上或非財產上之不利益，而是在於調整各種公法關係中因無法律上原因而發生財產變動之情形，用以回復至適法的狀態，因此，公法上不當得利，可說是與民法上不當得利具有相似功能的平行制度。
實務上在大法官釋字第515號解釋中確立了「公法上不當得利制度」，而相對於民法上之不當得利，亦衍生出應如何類推適用的問題。公法上不當得利之類型，依請求權及請求對象區分為「人民向行政主體請求者」、「行政主體相互間請求者」及「行政主體向人民請求者」三種。又於行政法律關係中，財產法性質之請求權，無論係公行政對人民或人民對公行政請求者，應皆有消滅時效之適用，始符合法律安定之要求。在各別法律內，如已明文規定「消滅時效」者，自有此一制度之適用；各別法律中無明文規定者，則依行政程序法第131條第1項之規定處理。而對於時效完成之法律效果，在學理上有認為僅發生義務人得拒絕履行義務之抗辯權者。惟為求公法法律關係之一致，以採本權利消滅之原則為宜。
在判斷是否成立公法上不當得利返還請求權之個案時，應先依其發生原因即財產變動、基於公法關係而生、無法律上原因等三項來逐一審查該個案是否的確成立公法上不當得利返還請求權。在經過審查後，確立為公法上不當得利返還請求權之類型後，更應判斷該案件是否為行政程序法第127條或稅捐稽徵法第28條、第38條第2項等特別規定之情形，如為所謂特殊公法上不當得利之情形，則依照特別法優於普通法之原則，優先適用相關之法規，以定其法律效果及與其他請求權相關之問題。如為一般公法上不當得利之情形，或係未經個別法律規定處理方式之特殊公法上不當得利態樣，則屬不成文之一般公法上不當得利，此時則依行政程序法之一般原理原則來處理相關問題。
First, this paper will discuss the topic about what is the meaning of returning unjustified enrichment in public law. The main occurrence of this topic has three aspects which are “the change of property”, “based on the relationship of public law” and “without legal reasons”. This phenomenon caused by the lack of legal reason and the request of changing property.
The purpose of establishing the system of returning unjustified enrichment in public law is to adjust the relationship of various public laws without using any legal reason of changing someone’s property situation, but to follow the appropriate situation of the law. Therefore, returning unjustified enrichment in public law and returning unjustified enrichment in civil code are parallel systems with similar functions.
The returning unjustified enrichment in public law can also divided into three categories based on the right of request or the request person. Those three types are "the request from people to administrative agency ", "the administrative agencies request to each other " and "the administrative agency request to people ".
Under the legal stability of the public law, the request of the property law claims that it has to obey the limitation period, neither it is administrative agency request to people nor people request to administrative agency.
While determine whether the returning unjustified enrichment is valid in the public law cases. The cases must go through all three steps which are “the change of property”, “based on the relationship of public law" and “without legal reasons ".
