Section 128 — Delegation of investigation duties
Statutory text (Thai original)
พนักงานสอบสวนมีอำนาจให้เจ้าพนักงานอื่นทำการแทน ดังต่อไปนี้
(๑) การใดในการสอบสวนอยู่นอกเขตอำนาจของตน มีอำนาจส่งประเด็นไปให้พนักงานสอบสวน ซึ่งมีอำนาจทำการนั้นจัดการได้
(๒) การใดเป็นสิ่งเล็กน้อยในการสอบสวน ซึ่งอยู่ในเขตอำนาจของตน ไม่ว่าทำเองหรือจัดการตามประเด็นมีอำนาจสั่งให้ผู้อยู่ใต้บังคับบัญชาทำแทนได้ แต่ทั้งนี้ เมื่อประมวลกฎหมายนี้หรือกฎหมายอื่นมิได้เจาะจงให้ทำด้วยตนเอง
English translation
An investigating officer has the power to have other officers act in his place, as follows: (1) for any matter in the investigation that is outside his jurisdiction, he has the power to send a commission to the investigating officer who has power over that matter to deal with it; (2) for any matter that is a minor part of the investigation within his jurisdiction, whether done by himself or dealt with under a commission, he has the power to order a subordinate to act in his place, provided that this Code or another law has not specifically required the matter to be done by himself.
This English translation is provided for reference only and has not yet been firm-verified — always rely on the Thai original.
Firm annotation
This provision opens Chapter 1 (Ordinary Investigation) and provides practical flexibility so that an investigation is not paralysed by jurisdictional limits or workload. Sub-section (1) allows a commission (a request to an out-of-area investigator), while sub-section (2) permits delegation of minor tasks to subordinates. The key limit is that neither route can be used where the Code or another law requires an act to be done personally, such as the settlement of certain traffic offences. The courts read the boundary strictly: an officer who acts outside his jurisdiction without a proper commission is not treated as acting on delegation, and that investigation can be unlawful.
Why this matters in practice
For a defendant, this section is worth checking whenever the investigation crossed police-station boundaries or was handled by a junior officer. If an officer investigated outside his own jurisdiction without a proper commission under Section 128(1), that part of the investigation can be unlawful, which may affect the prosecutor's power to file. Some acts, like certain traffic-fine settlements, must be done by the investigating officer personally and cannot be delegated at all. If jurisdiction of the investigating station is in doubt, a review with a lawyer is worthwhile; see how to choose a Thai lawyer.
Supreme Court decisions interpreting this section
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Supreme Court Judgment No. 2565/2565 (2022)
While some acts, such as settling a traffic offence, must be done personally by the investigating officer under Sections 38 and 144, Section 128(1) and (2) allow the investigator to request or order other officers to act in his place for certain matters.
The Supreme Court explained that a person who is not the investigating officer has no power to settle a traffic offence under the Land Traffic Act, since that is a personal power of the investigator under Sections 38 and 144. It noted, however, that Section 128(1) and (2) do allow the investigator to have other officers act in his place for certain matters, so the officer is not required to do everything personally.
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Supreme Court Judgment No. 12934/2553 (2010)
An investigator who acts outside his jurisdiction, where none of the exceptions in Section 18 paragraph two applies, cannot be regarded as acting in place of the competent investigator under Section 128, and his investigation is unlawful; a later investigation by the competent officer may cure the defect.
An investigator from one station had no power to investigate because the offence occurred outside his jurisdiction and no exception under Section 18 paragraph two applied, nor could it count as acting in place of the competent investigator under Section 128, so his questioning was unlawful. However, once the file was sent to the competent station, that officer re-investigated the witnesses.
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Supreme Court Judgment No. 371/2531 (1988)
Where an officer has no power to investigate because the offence occurred outside his jurisdiction and no exception under Section 18 paragraph two applies, his acts cannot be treated as acting in place of the competent investigator under Section 128, and the investigation is unlawful.
The Supreme Court held that a Bangkok-station officer had no power to investigate an offence that occurred in another district, and with no exception under Section 18 paragraph two, his questioning of the accuser's witness could not be treated as acting in place of the competent investigator under Section 128, so it was unlawful; the competent officer's later steps did not cure it because he had not treated his own part as complete.
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 an investigator delegate parts of an investigation?
Yes. Under Section 128 he may send a commission to an out-of-area investigator for matters outside his jurisdiction, and may order a subordinate to handle minor tasks within his jurisdiction.
Are there acts that cannot be delegated?
Yes. Where the Code or another law requires an act to be done personally, such as certain traffic-offence settlements, the investigating officer cannot delegate it.
What if an officer investigates outside his jurisdiction?
If he acts outside his jurisdiction without a proper commission under Section 128(1), it is not treated as valid delegation and that investigation can be unlawful.