Aircraft Flight Operation Regulations Article 280
An operator shall assign a co-pilot in the aircraft while operating an aircraft carrying passengers under IFR.
中文原文 航空器飛航作業管理規則第280條
航空器使用人依儀器飛航規則執行搭載乘員任務飛航時，應指派副駕駛員。
English translation for reference only; the Chinese text prevails. Not legal advice.