Section 120 — No prosecution without investigation
Statutory text (Thai original)
ห้ามมิให้พนักงานอัยการยื่นฟ้องคดีใดต่อศาล โดยมิได้มีการสอบสวนในความผิดนั้นก่อน
English translation
The public prosecutor is prohibited from filing any case with the court unless there has first been an investigation into that offence.
This English translation is provided for reference only and has not yet been firm-verified — always rely on the Thai original.
Firm annotation
This is a foundational rule of General Principles governing prosecution: a prior lawful investigation is a precondition to the public prosecutor's power to file a criminal case. The courts read it strictly, so an absent or fundamentally defective investigation is treated as no investigation at all, which deprives the prosecutor of authority to sue. It interacts closely with the investigation provisions that follow (Sections 121 onward) and with the definition of a valid complaint, and it has been applied even in specialised proceedings such as corruption cases through their governing statutes.
Why this matters in practice
For an accused person, Section 120 is a powerful defence: if the police never investigated the specific offence charged, or the investigation was so flawed it counts as none, the prosecutor has no power to file and the case can be dismissed. This is a point of public order the courts can raise on their own, even late in the case. If you suspect a charge was filed without a proper investigation, raise it early. A consultation with a Thai lawyer can help you test whether the investigation requirement was met.
Supreme Court decisions interpreting this section
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Supreme Court Judgment No. 3462/2567 (2024)
Section 120 applies even to inquisitorial corruption proceedings through the Act on Procedure for Corruption Cases B.E. 2559; if the prosecutor files without an investigation, or the investigation was unlawful and so counts as none, the prosecution is unlawful.
By plenary decision, the Supreme Court held that although corruption cases use an inquisitorial system in which the court may seek facts itself, they remain subject to the Criminal Procedure Code. Under Section 120, applied via Section 6 of the corruption procedure statute, filing without a prior investigation, or after an unlawful investigation that counts as none, renders the prosecution unlawful.
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Supreme Court Judgment No. 3277/2566 (2023)
The failure of the investigator to notify and investigate a particular charge bars only the prosecutor from filing that charge under Section 120; it does not limit the court's power at the trial stage to convict on that offence where the law otherwise permits.
The Supreme Court explained that an investigator's failure to notify and investigate a specific offence goes to the prosecutor's power to file under Section 120, which is a stage of filing, not to the court's power to adjudicate. The Court of Appeal for Specialised Cases therefore retained power to convict on the offence.
Curated decisions with case numbers verified against the Supreme Court database. English renderings are the firm's editorial translation for study.
Frequently asked questions
Can a prosecutor charge someone without a police investigation?
No. Under Section 120, a public prosecutor cannot file a criminal case unless the offence has first been investigated.
What happens if the investigation was defective?
The courts treat a fundamentally unlawful investigation as no investigation at all, so the prosecutor has no power to file and the case can be dismissed.
Does the investigation requirement apply to special courts?
Yes. Section 120 has been applied even in specialised proceedings, such as corruption cases, through the statutes that govern those courts.