According to the Monitoring Report 2025 Edition published by the Certified Public Accountants and Auditing Oversight Board on July 18, 2025, this report is created and published annually with the aim of providing information in an easily understandable format about the Board's monitoring activities and their results, targeting not only audit and accounting professionals but also market participants, students, working adults, and the general public. Starting from the 2024 edition, data renewal and addition of new items have been implemented.
The overview of the audit industry shows that approximately 78% of audit business revenue is concentrated among the Big Four audit firms. However, in recent years, the proportion of the Big Four in terms of audit attestation services, audit business revenue, etc., has been declining. For auditors of listed domestic companies, in fiscal 2024, the Big Four accounted for 58.1%, mid-tier firms 16.0%, and small firms 25.9%. This oligopoly trend is similarly observed in the United States (99%) and the United Kingdom (89%).
Regarding quality management systems, the response status to the revised "Quality Control Standards for Audits" in November 2021 is detailed. The revised standards introduced a risk-approach-based quality management system where audit firms themselves set quality objectives, identify and assess quality risks, and implement measures to address them, shifting from the previous establishment of certain quality control policies and procedures. Big Four and mid-tier audit firms have already applied these standards, and small and medium-sized audit firms are also developing systems for application from July 1, 2024.
The operational status of audit firms includes quality management measures such as regular verification of audit work, utilization of global reviews, and pre-audit opinion monitoring. Regarding auditor changes, there were 205 cases in June 2025, with an increase in movements from Big Four audit firms to mid-tier and smaller firms.
The article also mentions responses to environmental changes surrounding audits, including the growing role of small and medium-sized audit firms and trends in sustainability disclosure and assurance, as well as the latest institutional reforms.