With the recent adoption of International Financial Reporting Standards (IFRS), the regulatory approach to accounting principles has shifted from a rules-based to a principles-based framework. The previous rules-based standards presented specific accounting treatment criteria numerically or clearly enumerated detailed items. In contrast, principles-based standards present principles based on narrative criteria with minimal examples and guidelines for the application of the relevant standards, allowing accountants to exercise reasonable judgment in accounting treatment within the scope of these principles. This study examined the effects of the existing rules-based standards and the principles-based standards currently advocated by IFRS on the judgment of auditors who apply them. Additionally, the study tested the effect of the increased likelihood of regulatory review by supervisory authorities on auditors' judgment following the introduction of the new accounting system. The research employed an experimental method, and data were collected through questionnaires. Experimental participants were designed to make accounting treatment decisions for the audited company in a hypothetical audit situation involving lease classification. The treatment variables were designed as accounting principle regulatory approach (principles-based vs. rules-based) and review likelihood (high vs. low) in a 2×2 between-subjects factorial design. For the principles-based standard, the K-IFRS provisions on leases were presented, and for the rules-based standard, the general corporate accounting standards on leases were presented. The dependent variable was measured as the auditor's lease classification decision. The experimental subjects of this study were limited to certified public accountants currently affiliated with the Big Four accounting firms with more than three years of audit experience. The data analysis results showed that auditors made more client-favorable decisions (aggressive decisions) under ambiguous principles-based standards than under clear rules-based standards. However, when the likelihood of regulatory review increased, this decision-making tendency decreased, and it decreased more under the principles-based approach, although the results were not statistically significant. Therefore, under principles-based standards such as IFRS, supplementation of IFRS standards is required in a direction that can reduce auditors' aggressive decision-making. Furthermore, an examination of the degree of consensus among experimental participants' responses revealed that under principles-based standards, the possibility of auditors choosing different accounting treatments for the same transaction increased. Moreover, even when the likelihood of supervisory authority review was increased, the possibility of different accounting treatments did not decrease. These research findings suggest that under IFRS, a principles-based framework, the possibility of different interpretations of ambiguous standard content by accounting decision-makers may increase. Therefore, there is a need to supplement the standards in a direction that can reduce the tendency for auditors to apply different accounting treatments to the same transaction under a principles-based framework such as IFRS.