Half-mast Implementation Regulations Article 5

In case of heavy casualties resulting from a natural or man-made disaster, according to the disaster category, the central competent authorities of disaster prevention and rescue operations shall make an application to the Executive Yuan for half-mast determination; in the case of an international disaster event, the application shall be made by the Ministry of Foreign Affairs to the Executive Yuan.
中文原文 國旗下半旗實施辦法第5條
因天然或人為災害造成重大傷亡時,依災害類別,由中央災害防救業務主管機關報請行政院決定下半旗。
但屬國際災害事件者,由外交部報請行政院決定。

English translation for reference only; the Chinese text prevails. Not legal advice.