Section 49. In the case where a taxpayer deriving income does not file a tax return, or the assessment official considers that he underreports the amount of his taxable income, the assessment official with the approval of the Director-General shall have the power to determine the amount of his net income on the basis of the money or property owned or possessed by such taxpayer, his expenditure or standard of living or his behavior, or the income statistics either of the taxpayer or of other persons carrying on a similar business. The official shall make an assessment accordingly and give the taxpayer a notice of the amount of tax payable. In this respect, the provisions of Sections 19 through 26 shall apply mutatis mutandis.