How to say "illegal" in Japanese

• 違法いほうCommon
  na-adjective, no-adjectiveA plain, common word for 'illegal': something that is against the law. Typical in legal and news contexts, such as reports of unlawful conduct.
  あそこに駐車ちゅうしゃするのは違法いほうです。↗

  It is illegal to park a car there.
• 不法ふほうCommon
  na-adjective, nounVery close to 違法 in meaning; the two are near-synonyms, and in most contexts either will serve. Common in news reporting on unlawful acts.
  皆みなが不法ふほう外国人がいこくじんに対たいして敵対的てきたいてきな態度たいどを取とった。↗

  Everybody took a hostile attitude toward illegal aliens.
• 非合法ひごうほうCommon
  na-adjective, no-adjectiveCovers 'illegal; unlawful'. Built on 合法 'legal' with the negative 非, it states outright that something is not legal — suits discussion of activities or organizations that operate outside the law.
  奴隷どれい制度せいどが非合法ひごうほうとなってから１世紀せいき以上いじょうになる。↗

  It is over a century since slavery was made illegal.
• 裏口うらぐちJLPT N2approx.Common
  no-adjectiveReally the word for a back or rear entrance; as 'illegal' it fits only the figurative backdoor sense of unauthorized access or dealings, and is seldom the word you want otherwise.
  彼かれは裏口うらぐち入学にゅうがくした。↗

  He bought his way into college.
• アングラCommon
  prenominal modifierAn abbreviation referring to the underground: unofficial, illicit activity such as an underground economy or press, or black money. Use it when 'illegal' carries that underground flavor, not as a general word for lawbreaking.

For plain "illegal", 違法 and 不法 are close synonyms and either will do; 非合法 says something is simply not legal; 裏口 and 「アングラ」 work only for backdoor and underground senses.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 違法いほうillegal
• 不法ふほうunlawful
• 非合法ひごうほうillegal
• イリーガルillegal
• 掟破りおきてやぶりagainst the rules
• ダーティーdirty
• 反社会的はんしゃかいてきantisocial
• 地下ちかunderground (activities, movement, etc.)
• 裏口うらぐちunauthorized
• アングラunderground (economy, press, etc.)
• アンダーグラウンドunderground (economy, press, etc.)