Section 121 — Investigator power and complaint requirement
Statutory text (Thai original)
พนักงานสอบสวนมีอำนาจสอบสวนคดีอาญาทั้งปวงแต่ถ้าเป็นคดีความผิดต่อส่วนตัว ห้ามมิให้ทำการสอบสวน เว้นแต่จะมีคำร้องทุกข์ตามระเบียบ
English translation
An investigating officer has the power to investigate all criminal cases. However, in the case of a compoundable offence (an offence against a private person), an investigation is prohibited unless there is a complaint lodged in accordance with the regulations.
This English translation is provided for reference only and has not yet been firm-verified — always rely on the Thai original.
Firm annotation
This General Principles provision defines the scope of the investigator's authority and its single key limit. For public (state) offences the officer may investigate on his own initiative, but paragraph two makes a valid complaint an absolute precondition to investigating a compoundable offence. Because a lawful investigation is required before the prosecutor can file (Section 120), a defect here is fatal: with no valid complaint there is no power to investigate, and with no valid investigation the prosecutor has no power to sue. This is why the courts treat the question, often raised together with the meaning of injured person under Section 2(4), as one of public order.
Why this matters in practice
For compoundable offences such as embezzlement, fraud, or defamation, everything can turn on whether the right person lodged a valid complaint. If the complainant was not the true injured party, the investigation is void and the whole prosecution collapses, and the courts can raise this on their own even if the defendant never argued it. Victims should make sure the complaint is filed correctly and by the right person, and defendants should check whether it was. Getting this right often needs a lawyer, and our guide on how to choose a Thai lawyer can help.
Supreme Court decisions interpreting this section
-
Supreme Court Judgment No. 626/2565 (2022)
Where an investigator has no power to investigate a compoundable offence under Section 121 paragraph two, the public prosecutor consequently has no power to file; a lower court's ruling on this point may require the case to be remanded so appeals are decided in proper order.
The Court of Appeal (Region 5) dismissed the case on the ground that the investigator had no power to investigate under Section 121 paragraph two, without first ruling on the parties' other appeals. The Supreme Court found this could improperly restrict the parties' right of appeal and remanded the case for fresh consideration.
-
Supreme Court Judgment No. 404/2565 (2022)
A person who is not the injured party by law under Section 2(4) has no right to lodge a complaint for a compoundable offence such as embezzlement; the investigator therefore has no power to investigate under Section 121 paragraph two, and the prosecutor has no power to file. This is a public-order point the court can raise on its own.
Because the person who lodged the complaint was not the injured party by law, he had no right to complain about the alleged joint embezzlement, a compoundable offence. The investigator thus had no power to investigate under Section 121 paragraph two and the prosecutor no power to file. The Supreme Court raised the point on its own even though most defendants had not appealed it.
-
Supreme Court Judgment No. 3578/2564 (2021)
Where the true injured party has not lodged a complaint for a compoundable offence such as joint mischief (damage to property), the investigator has no power to investigate under Section 121 paragraph two, the prosecutor has no power to file, and the victim cannot claim compensation in that count.
The true victim of the property damage had not lodged a complaint, so the investigator had no power to investigate that charge under Section 121 paragraph two. The Supreme Court held the prosecution of that count was unlawful and the victim could not obtain compensation for the motorcycle repair in that count.
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 police investigate a compoundable offence without a complaint?
No. Under Section 121 paragraph two, an investigator may not investigate a compoundable offence unless the injured person has lodged a proper complaint.
What happens if the wrong person filed the complaint?
If the complainant was not the true injured party, the investigation is unlawful and the prosecutor has no power to file, so the case can be dismissed.
Do state offences also need a complaint?
No. For public (state) offences the investigator may act on his own initiative under Section 121 paragraph one; the complaint requirement applies only to compoundable offences.