The convenience and efficiency of mobile cranes are expanding their applicability in industrial sites, but fatal accidents continue to occur as their use increases. There were 56 cases in South Korea from 2015 to 2019, killing 59 workers. To accurately investigate the cause of a fatal accident, accident investigation reports were used. Since they are used not only as the cause of the accident but also as a result of judicial treatment, only direct causes are mentioned. Thus, indirect causes in this study were separately analyzed to induce a complex cause analysis. The man-made, management, economic, physical, political, and social (MEPS) analysis method, developed by the National Institute of Disaster in South Korea, is a type of root cause analysis (RCA), used to derive the fundamental causes of various types of disasters, mainly social ones. The complex causes of fatal accidents were analyzed by applying a modified MEPS method to mobile cranes. The MEPS method investigated three categories, namely man-made, management, and physical factors, among six categories and a newly established level four, to find the root cause of fatal accidents. The analysis results showed that violations of procedures and regulations were the most frequent causes in the man-made factors. A lack of general and special safety education was the most common cause in the management factor, and the overturning, falling, and jamming of the mobile crane were the most frequent causes in the physical factor.