This study investigated the pressure difference distribution and the flow characteristics among room, ancillary room, and stair case by carrying out the numerical simulations on the air flow inside the pressurized air supply smoke control system. Numerical simulations were conducted to analyze pressure and velocity distribution of compartments by pressurized air supply for the air-leakage test facility which was built to measure the effective leakage area. In this study, the leakage of air was considered by locating the narrow slit onto fire door and window of room. Simulated results using this method precisely followed the previous experimental results for the pressure differences between the stair case and ancillary room. Predicted results showed that the local leakage of air rarely affected the overall flow pattern and pressure distribution. Although the average velocity over the door between room and ancillary room satisfied the regulation for fire safety, it was certified the unsafe outflow to ancillary room could be occurred in the local position such as the upper part of the door.