Regulations on internal control systems became a part of the Act on External Audit of Stock Companies in late 2003, making them official from its temporary status. Since then, there have been numerous studies examining the effect of the introduction of internal accounting control systems and the effectiveness of internal control systems in reducing the occurrences of material weaknesses. While prior studies focus on the effect of the introduction of internal accounting control systems, this study separates the characteristics of internal accounting control into quantitative and qualitative factors and examines the effectiveness of internal accounting control systems in reducing the occurrences of accounting errors. Korea is the only country that discloses information about the characteristics of investment in human resources in internal accounting control systems to provide better understanding for interested outside parties to judge the effectiveness of internal controls over financial reporting. Firms with more human resource investment in internal controls could manage their internal accounting control more effectively. Empirical results are as follows: the firms with a higher proportion of internal control personnel to employees show a lower probability of accounting errors occurring. This result implies that when a firm has a certain number of internal control personnel, it can appropriately segregate their duties which leads to timely review and proper monitoring of accounting procedures,resulting in more effective internal controls. When the average working experience of internal control personnel is longer, there is a lower probability of accounting errors occurring. This result suggests that there is an increase in the learning effect when internal control processes are done repetitively. This effect could be from a better understanding of the firm and more effective controling over accounting errors by experienced personnel. However, the test on internal control personnel who possess CPA licenses does not show a significant relationship with the effectiveness of internal accounting control. This insignificant result may be because only a few firms have CPAs as members of internal control personnel.