Internal Accounting Control System(IACS) is a regulation adopted to ensure the reliability of accounting information prepared and disclosed in accordance with GAAP. Korean external audit law requires external auditors to provide review opinion on whether IACS of an company is properly designed and effectively operates, and attach it to the audit report. If review opinion on IACS, which was adopted to ensure the reliability of accounting information, is useful, outside information user will consider the opinion upon decision making. Therefore, to validate the effectiveness of review opinion on IACS, this paper examined whether it affects an company's cost of capital. In addition, this paper examined whether there are differences in impacts on cost of capital between companies represents existence of material weakness, and companies with disclaimer of opinion. Lastly, this paper examined how a change in review opinion compared to the prior year affects the cost of capital. According to the results, companies with non-unqualified opinion incurred higher cost of capital compared to companies with unqualified opinion. Second, among the companies with non-unqualified opinion, companies with disclaimer of opinion incurred higher cost of capital compared to companies represents existence of material weakness. Lastly, companies with negative change in review opinion incurred higher cost of capital compared to companies with improved review opinion. These results suggest that outside information users take review opinion on IACS as useful information, and conclude that accounting information of companies with non-unqualified opinion implies higher information risk, consistent with the purpose of IACS, which is designed to ensure the reliability of accounting information.