Aircraft Flight Operation Regulations Article 335

No pilot-in-command (PIC) of an airplane having a seating capacity of more than 19 passengers may permit a passenger to stow baggage aboard that airplane except:1. In a suitable baggage or cargo storage compartment, or as provided in Article 336; or2. Under a passenger seat in such a way that it will not slide forward under crash impacts severe enough to induce the ultimate inertia forces.
中文原文 航空器飛航作業管理規則第335條
座位數超過十九座之飛機應符合下列條件之一,機長始得准許乘員將行李置於機上:
一、具有適當之客艙行李櫃、貨物存放隔艙或符合第三百三十六條之規定。
二、於遭受撞擊時所產生之型別檢定最大慣性力及側面移動力時,置於乘員座椅下方之行李,不致前移或側移。

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