2020 年5 月29 日，我國大法官會議以第791 號解釋宣告了刑法通姦罪違憲，直接將爭議多年的通姦罪一舉「除罪化」，在我國由來已久的通姦罪，自此走入歷史。本號解釋之重要論據係憲法第23 條之「比例原則」。雖然比例原則在行政法領域之審查運用甚廣，其模式也日益定型，但就刑事法方面而言，基於刑事法與行政法之作用不同，尤其是刑事實體法之獨特性、嚴厲性，大法官在違憲審查上，與審查國家其他行為時理應有所不同。本文先行回顧大法官解釋以往之操作方式，繼而針對本號解釋之理由書及協同、部分協同、不同理由書等加以分析，認為大法官以往對於刑事實體法之審查上存在諸多疑慮，本號依然欠缺明確且具體之理由，其直接宣告違憲，不無侵害立法權之虞，而且對於適合性、必要性、狹義比例原則等子原則之說明，並未本於刑事實體法之特殊性而調整，驟然以宣告違憲之理由直接除罪化，一旦在未來形成慣例，有除罪化爭議之其他刑事實體法，恐將面臨不當之違憲審查。

On May 29, 2020, the TCC (Taiwan Constitutional Court) renders the J.Y. Interpretation No. 791, holding the Article 239 of the Criminal Code unconstitutional. The provision shall become null and void immediately after the announcement of this interpretation. In the interpretation, the TCC held the adultery crime is unconstitutional by the principle of proportionality. Although the principle of proportionality is usually be taken as the standards of judicial review, and it is becoming more and more finalized. But the basis of criminal law and administrative law is different, especially the uniqueness, strictness of criminal law. The standards of the principle of proportionality should be different from the others. The study confirm the significance of the principle of proportionality—the due purpose, necessary means, and proper restrictions required by the previous J.Y. Interpretations concerned with the Criminal Code. And the finding is some of them—especially the No. 791, violate the principle of separation of powers, and try to make some suggestions for reference.