The Commercial Port Law Article 20
A ship shall hoist the ROC national flag, the national flag of her registration port, and the call sign flags, while entering or departing from a port.
中文原文 商港法第20條
船舶入港至出港時，應懸掛中華民國國旗、船籍國國旗。
English translation for reference only; the Chinese text prevails. Not legal advice.