Regulations Governing the Withholding of Tax on Income from Salaries and Wages Article 6

As to salaries paid monthly whose recipient fails to submit Tax Exemption Declarations in accordance with Paragraph 1, Article 3, 5% of the total monthly payment shall be withheld in accordance with Item 2 of Subparagraph 1 of Paragraph 1 of Article 2 of Standards of Withholding Rates for Various Incomes.As to wages for part-time jobs and salaries not paid monthly, tax withholders shall withhold 5% of the total payment.
中文原文 薪資所得扣繳辦法第6條
按月給付之薪資,受領人未依第三條第一項規定填報免稅額申報表者,應依各類所得扣繳率標準第二條第一項第一款第二目規定,按全月給付總額扣取百分之五。
兼職所得及非每月給付之薪資,扣繳義務人按給付額扣取百分之五。

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