Most cases of Middle East Respiratory Syndrome (MERS) infection in Korea (outbreak: May 11-July 4, 2015) occurred in hospital settings, with uncertain transmission modes in some cases. We performed an in-depth investigation epidemiological survey on the 178th case to determine the precise mode of transmission. A 29- year-old man living in Pyeongtaek presented on June 16 with a febrile sensation, chills, and myalgia. Upon confirmatory diagnosis on June 23, he was treated in an isolation room and discharged on July 2 after cure. An epidemiological investigation of all possible infection routes indicated two likely modes of transmission: exposure to MERS in Pyeongtaek St. Mary's Hospital during a visit to his hospitalized father (May 18-29), and infection through frequent contact with his father between the latter's referral to Pyeongtaek Good Samaritan Bagae Hospital for treatment without confirmatory diagnosis until his death (May 29-June 6). Although lack of clear proof or evidence to the contrary does not allow a definitive conclusion, all other possibilities could be excluded by epidemiological inferences. While it is impossible to trace back the modes of transmission of all cases in a large-scale outbreak, case-by-case tracking and isolation of infected individuals and those in close contact with them is important in preventing the spread. Efforts should be made to establish a methodology for rapid tracking of all possible contacts and elimination-based identification of the precise modes of transmission.