Due to the recent outbreaks of infectious diseases, such as severe acute respiratory syndrome (SARS), Influenza and Ebola, isolation facilities have played an important role to prevent infectious diseases from spreading at initial stage. An isolation ward is a facility to isolate patients physically and to care them safely. One way to isolate a patient physically is to build a negative pressure isolation facility. However, unexpected failure or misuse of such facility makes it difficult to maintain negative pressure and eventually causes secondary infection, leaking the infectious pathogen to outside of the isolation ward. This study identifies the amount and velocity of leakage air from a patient ward by tracer gas experiment under abnormal operations of an isolation facility. In addition, computational fluid dynamics (CFD) allowed us to observe the outflow mechanism of pollutant. The results show that abnormal operations of a facility spreads pathogens to neighboring areas immediately and timely actions should be prepared against them.