2008年釋字第647號公布，針對稅法上未賦予「事實上夫妻」法律上配偶相同待遇，是否牴觸憲法做出解釋。其理由書謂無配偶之人相互間主觀上具有如婚姻之共同生活意思，客觀上亦有共同生活事實之異性伴侶，雖不具法律上婚姻關係，但既與法律上婚姻關係之配偶極為相似，如亦有長期共同家計之事實者，因具婚姻關係配偶之相似性，立法機關得在無損於婚姻制度或其他相關公益之前提下，給予適度之法律保障。之前於2007年通過民法修正，將婚姻制度由過去「儀式婚」改為「登記婚」，不過社會上迎娶的習俗並未改變，男女只踐行婚禮而未為結婚登記者，其法律地位如何。而各國同居人口不斷增加，各國法制莫不積極因應，據調查國內目前同居人口亦高達44萬人以上，因此外國法制確有參考的實益。礙於篇幅及文獻限制，僅就人民往來頻仍之美國、率先以民法典規範之法國、以裁判先例處理之日本、文化背景近似之中國大陸等地介紹之。 　　本論文分六章，第一章介紹研究動機及研究方法，研究之動機主要是大法官釋字第647號解釋之立法指示、2008年結婚方式修正後預立補強之道。其研究方法係以法解釋學為基礎，透過文義、體系、立法、目的及合憲解釋等方法認識各國法制，再進行法制間的比較、歸納、演繹等分析，期能借鑑最適我國情之法制。第二章為各國事實婚之用語、規範類型與發展介紹。以較宏觀的角度檢視各國事實婚法制，包括其用語、規範的方式及發展背景，各國對於事實婚的表達未必一致，如何擷取具有參考價值的法制，成為研究事實婚的第一個課題。其次規範的方式各國間亦不相同，有以成文法規範之，亦有以裁判先例處理之，甚至解決的重心亦不相同；而各國事實婚形成的背景，更是研究外國法制所不可或缺者。第三章討論事實婚保護之法理，一般認為事實婚的保護，將有害婚姻憲法權威地位，承認事實婚之地位必須具有強而有力的理由。目前雖然沒有事實婚的國際條約，不過婚姻自由、家庭保護、平等不歧視、兒童保護等國際法上普世原則的尊重，亦間接保護事實婚。而已進行事實婚保障之國家，其調整憲法上婚姻地位獨尊的論述，亦值得參考。第四章、第五章則具體分析美、法、日、中國大陸等地關於事實婚之規定，包括美國法律協會的家庭解消建議、法國民法Pacs制度、日本內緣關係、中國大陸之（非法）同居關係等，同時分析我國司法實務的判決。 　　第六章為結論，本論文肯定事實婚保護之必要性，強調共同生活事項類推婚姻法效果的合理性，並根據此結論提出建議，包括援引外國立法例的調整、國內實務解釋的改進、立法雛議與男女共同生活契約範本的提出，期能增進男女地位的公平與弱勢伴侶之保護。 / The reasoning of J. Y. Interpretation no.647 has pointed out ”Those unmarried companions of opposite sex with a subjective intent to live together like a married couple and the objective fact of cohabitation do not have a matrimonial relation in law, the relationship between them is very much similar to the relationship of legally married husband and wife, and the provision at issue, which does not provide for gift tax exemption for mutual gifts between such companions, would seem susceptible to a doubt of violating the principle of equality provided that there is in addition a fact of longtime sharing of the livelihood between such couples. As for those cohabited companions with similarity to a legal marriage, the Legislature may, taking into considerations the constitutional protection of fundamental rights of the people as well as changes in the social condition and cultural development, give them adequate legal protection to the extent not disparaging marriage institution and other public interests.” Though the formal requirement of validity of marriage in Civil Law in Taiwan has changed from ceremony to registration since 2007, it is generally recognized that a valid marriage should be done on the basis of a marriage ceremony, by which a gap between citizen’s perception and legal norms has come out and the commence of de facto marriage becomes inevitable. Furthermore, the amounts of cohabitation in Europe and the United States have increased constantly so that the legislation of long-term de facto marriage has more or less been made in these countries. In contrast, though the amounts of cohabitation in Taiwan is estimated to exceed 0.4 million, the judicial practices for the legal issue of cohabitation are not coherent among cases in courts. Accordingly, this thesis focuses on the comparison of legislation for de facto marriage among several countries with a view to providing some suggestions to the law making and judicial practice in Taiwan. Firstly, the motivation and methodologies are introduced in chapter one. As to methodologies, this thesis starts from the interpretation of law; and comparative, inductive and deductive approaches are used for the research of legal systems in some countries. Secondly, this thesis analyses the terminology, types of legislation and development of de facto marriage in chapter two. Inter alia, it includes the definition of de facto marriage made by Chinese, Japanese and English-speaking scholars and the models of legislation for the protection of de facto marriage in some countries. In addition, it elaborates the background of de facto marriage and its evolution of legal regime in the United States, France, Japan and China. In chapter three, it deliberates the legal basis of de facto marriage. It commences from the stipulation of international treaties, in particular the European Convention on Human Rights, and the judicial practices of the European Court of Human Rights on the issues of freedom of marriage, protection of family rights, non-discrimination and the best benefits for children. It then compares the pros and cons for the protection of de facto marriage so that its necessity and possibility could be established at the constitutional level. In chapter four, this thesis elaborates the requirements of de facto marriage in United States, France, Japan and China, for instance, the Recommendations of Family Dissolution made by the American Law Institute (ALI), the Pacs in France, “Nai En” system in Japan and the (illegal) cohabitation in China. Furthermore, it also analyses Taiwan’s legal system of de facto marriage. This thesis then discusses in chapter five the legal effects of de facto marriage in the United States, France, Japan and China, and their impact on Taiwan’s related legislation. Finally, it concludes in chapter six by recognizing the necessity to protect de facto marriage and the legitimacy to analogize the legal effect of marriage law to common life matters. Accordingly, this thesis suggests that foreign legislations be ushered in for the amendment of domestic laws; the improvement of the interpretation of domestic judicial practices be made, and model contract for cohabitation be proposed in order to improve the fair status between male and female and the protection for the weaker one of the couple.