本文所探討者，為行政機關於個案中面臨新法秩序得否溯及適用疑義時，可供其判斷之若干原則。法令不溯及既往原則與信賴保護原則密切相關，其具憲法層次的效力，不但拘束行政與司法機關，亦拘束立法機關。本文所探討的「法令變遷」概念，係兼指「形式上」與「實質上」的法令變遷。亦即除法律或行政命令經法定程序而制訂、修正或廢止，所引起的法秩序變動者外；也包含法令在形式上未經修訂，但行政機關見解改變，作出不同於以往的解釋之情形。傳統學說所為「真正溯及既往」與「不真正溯及既往」之區分仍有其價值；反之「法律效果之溯及生效」與「法律事實之回溯連結」的概念，因僅由法令生效時間與法令時間上的適用範園為界定概念的要素，忽略案例事實的具體態樣，對問題的解決不具有實質意義。本文認為應由實質面向來界定「法令接續適用」典「法令溯及適用」二概念，依新法秩序實際上所造成的影響來決定信賴保護的手段；且應謹慎使用「本案並非法律溯及適用」等類似表達，以免造成「此時無須給予信賴保護」的誤解。執法者於判斷某項法令得否溯及適用時應考量的因素計有：(1)「新法秩序」是否對個人產生不利之變化；(2)信賴基礎的態樣；(3)人民之信賴表現；(4)當事人之信賴是否值得保護；(5)當事人所已取得之法律地位；(6)「法令溯及適用」對於當事人權利影響之程度；(7)溯及適用所能獲致之公益效應；(8)新法秩序之施行有無過渡期間。

This article explores how administrative agencies determine whether to apply new laws retrospectively. The prohibition of ex post facto laws is in high connection with the protection of reliance interest. Both are significant legal principles with constitutional status and therefore bind the administrative agencies, the courts, and the legislature alike. By “change of laws", the author indicates both the “formal" and “substantive" changes of laws. In addition to the enactment, amendment, or repeal of legislations or regulations through formal processes, it also includes the case that the administrative agency changes its previous interpretations of laws. In this vein, the traditional dichotomy between the “real retrospective application" and the “non-real retrospective application" remains making sense. To the contrary, the arguable theory distinguishing “retrospective applications of legal effect" from “retrospective applications of legal facts" is too formalistic. It ignores the specific facts in concrete cases, and thus has no practical benefit. This article advocates for defining the notion of “retrospective application of laws" in terms of substantive respects, i.e. to determine the means to protect the reliance interest in light of the practical implications led by the new laws. Also, in order to avoid misunderstanding that no reliance interest exists, certain kinds of descriptions such as “there is no retrospective application of laws" should be employed more prudently. In figuring whether a specific legislation or regulation could be applied retrospectively, the agency must take the following factors into account: (1)whether the new laws have negative impact on individuals; (2) the pattern of the cause of the reliance; (3) the specific performance because of the reliance; (4) whether the reliance is deserving of legal protection; (5) the legal status that the individual has already got; (6) the degree that the retrospective application makes impact on the party; (7)the public interest led by the retrospective application; and (8) whether there is a grace period for the new law to take into effect.