Prohibition of ex post facto laws

This principle states that a newly enacted law (ex post facto law) should not be applied retroactively to laws that were enacted before the law was enacted. This principle is also called the principle of non-retroactivity of laws. However, the purpose of this principle is applicable when an ex post facto law causes disadvantage or inequality to the parties involved, and it should not be understood in a formal sense. This principle is particularly important in the field of criminal law as a derivative principle of the principle of criminal legality. In response to this, Article 39 of the Constitution stipulates that "No person shall be held criminally responsible for any act that was lawful at the time it was committed." However, in the past, the application of this principle was limited to (1) substantive criminal laws and did not extend to the fields of criminal procedure law and criminal administration law (criminal treatment law), and (2) did not apply to cases where a court changes a precedent to the defendant's disadvantage.

In response to this, it has been argued that the intent of the prohibition of ex post facto laws should also be taken into consideration when making changes to laws or precedents that are detrimental to the defendant in terms of whether a crime has been committed or the determination of the sentence (in particular, (2) has been established in American precedents).

[Tetsuro Nawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

新たに制定された法律（事後法）は、その制定以前にさかのぼって適用してはならない、という原則。法律不遡及(そきゅう)の原則ともいう。ただ、この原則の趣旨は、事後法により関係者に不利益や不平等を生じる場合に妥当し、これを形式的に理解してはならない。この原則は、とくに刑事法の領域において罪刑法定主義の派生的原理として重要な意義をもつ。これを受けて、憲法第39条は、「何人(なんぴと)も、実行の時に適法であつた行為……については、刑事上の責任を問はれない」と規定している。ただ、この原則の適用にあたって、従来、(1)実体的な刑罰法規に限られ刑事手続法や行刑法（犯罪者処遇法）の領域には及ばない、(2)判例の不利益変更、すなわち、裁判所が判例を被告人の不利益に変更する場合には及ばない、とされてきた。

　これに対して、実質的に、犯罪の成否や刑の量定にとって被告人に不利益をもたらす法令や判例の変更についても、事後法の禁止の趣旨を考慮すべきである、という考え方が主張されている（とくに、(2)については、アメリカの判例で確立している）。

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