This study mathematically analyzed the relationships among pre-tax income standardized by equity (pre-tax income ratio), corporate tax differences standardized by equity (corporate tax difference ratio), and effective tax rates using a formal model, and then empirically verified these relationships. The corporate tax difference refers to the tax subsidy proposed by Wilkie (1992). The empirical analysis results support the model-based analysis, which can be summarized as follows. First, in the regression analysis of effective tax rates on pre-tax income ratios, the sample with positive corporate tax differences (receiving tax benefits) is explained by a nonlinear natural logarithmic model, while the sample with negative corporate tax differences (receiving tax disadvantages) is explained by a nonlinear irrational fraction model. The correlation and regression analyses for the full sample showed that effective tax rates are positively related to pre-tax income ratios, but no specific functional form was identified. Second, in the regression analysis of corporate tax difference ratios on pre-tax income ratios, the sample with positive corporate tax differences is explained by an upward-sloping linear model, while the sample with negative corporate tax differences is explained by a downward-sloping linear model. The correlation and regression analyses for the full sample showed that the corporate tax difference ratio has a positive linear relationship with the pre-tax income ratio. Third, in the analysis of the relationships among pre-tax income ratios, corporate tax difference ratios, and effective tax rates, regression analysis and portfolio analysis with the effective tax rate as the dependent variable after controlling for the corporate tax difference ratio showed a positive relationship between pre-tax income ratios and effective tax rates. However, the analysis of the signs of changes in the three variables and the regression analysis with changes in effective tax rates as the dependent variable showed a negative relationship between pre-tax income ratios and effective tax rates.