No country completely prohibits auditors from simultaneously providing audit services and non-audit services, but major countries permit the provision of non-audit services within a limited scope where the potential for conflicts of interest is minimal. Prior studies have shown conflicting results: some negative findings suggesting that the simultaneous provision of audit services and non-audit services impairs auditor independence and reduces audit quality, and other findings indicating no relationship between the simultaneous provision of non-audit services and audit quality. This study selected a sample of 11,601 firm-years from 2002 to 2013 for firms listed on the Korea Exchange (KRX) and KOSDAQ markets as of the end of 2013, and conducted empirical analyses examining the effect of auditor type on the relationship between the simultaneous provision of non-audit services by the same auditor and audit quality measured by audit fees, differentiated by types of non-audit services. This study classified auditor types not only by Big4 auditor status but also by "One firm" auditor status (auditors among the "One firm" type other than Big4, which integrate and manage funds, accounting, and engagement acceptance at the firm level) and by auditor groups based on the Securities and Futures Commission's auditor designation criteria. Additionally, non-audit services were categorized into tax services and non-tax services for analysis. The main empirical findings of this study are as follows. First, the simultaneous provision of audit services and non-audit services was found to increase audit fees, indicating that, contrary to general concerns, the simultaneous provision of audit and non-audit services has a positive effect on audit quality. Second, the effect of the simultaneous provision of audit services and non-audit services on audit fees differed by auditor type. When the same auditor provided non-audit services, audit fees increased for Big4 and One firm auditors; however, for accounting firms classified as the largest auditor group (GR1) under Securities and Futures Commission designation criteria other than Big4 (DSS) and for accounting firms in the second group (GR2), no relationship with audit fees was found. The finding that audit fees were unrelated for DSS and GR2, while they increased for Big4 and One firm auditors, implies that when Big4 and One firm auditors simultaneously provide audit services and non-audit services, it has a positive effect on audit quality. Third, the effect of auditor type on the relationship between the simultaneous provision of audit services and non-audit services and audit fees differed by the type of non-audit service. Big4 auditors showed increased audit fees when providing both tax services and non-tax services, while One firm auditors showed increased audit fees when providing tax services. The results of this study suggest that, to enhance auditors' quality control, policy measures are needed not simply to increase the size of accounting firms but to support and incentivize the development of "One firm" type auditors that integrate and execute fund management, accounting, and engagement acceptance at the firm level. Furthermore, the findings imply that differentiated policies are necessary depending on the type of non-audit service (tax services versus non-tax services).