Translation guide
In Japanese, expressing 'secret investigation' depends on the context: a formal inquiry, undercover work, private sleuthing, or internal corporate probe. The most common and versatile term is 内偵 (naitei), but other words like 秘密捜査 (himitsu sōsa) and 潜入捜査 (sennyū sōsa) are used for specific types of investigations.
To refer to a discreet inquiry or surveillance, often by police or private investigators, without the subject's knowledge.
The most common term for a secret investigation, especially by police or detectives. It implies covert surveillance and information gathering.
警察は容疑者を内偵している。
The police are conducting a secret investigation on the suspect.
Literally 'secret investigation', used for formal undercover operations, often by law enforcement. More explicit than 内偵.
秘密捜査で違法薬物の取引を突き止めた。
Through a secret investigation, they uncovered an illegal drug deal.
Undercover investigation where an officer infiltrates an organization. Common in crime dramas.
潜入捜査官が組織に潜入した。
An undercover agent infiltrated the organization.
Informal term for probing or snooping around, often used in everyday contexts rather than official investigations.
彼は会社の内情を探りを入れている。
He's secretly probing into the company's internal affairs.
To describe a confidential inquiry within a company, such as into misconduct or compliance issues.
Internal investigation, often confidential. Not necessarily 'secret' but implies a non-public inquiry.
不正の疑いで内部調査が行われた。
An internal investigation was conducted due to suspicion of misconduct.
Top-secret investigation, emphasizing high confidentiality. Used in corporate or governmental contexts.
極秘調査の結果、不正が明らかになった。
The top-secret investigation revealed the misconduct.
To refer to an individual hiring a detective or conducting their own discreet inquiry, e.g., into a spouse's affair.
Background check or personal investigation, often done discreetly. Common in pre-marital checks or suspicion of infidelity.
彼は結婚前に相手の身辺調査を依頼した。
He requested a secret investigation into his partner's background before marriage.
Investigation into someone's conduct or behavior, often by a private detective. Implies secret surveillance.
探偵に夫の素行調査を頼んだ。
She hired a detective to secretly investigate her husband's behavior.
To describe secret investigation in the context of spying or national security.
Intelligence activities, espionage. Formal term for secret information gathering by state actors.
その国は周辺国への諜報活動を強化した。
The country intensified its secret intelligence activities against neighboring nations.
Spy activities, espionage. More colloquial than 諜報活動.
彼はスパイ活動の容疑で逮捕された。
He was arrested on suspicion of espionage.
内偵 is the broadest term for covert investigation, often surveillance without direct contact. 秘密捜査 is a formal secret investigation, which may include undercover work. 潜入捜査 specifically means infiltration undercover operations.
The direct translation '秘密の調査' is not idiomatic. Use the terms above depending on context.