Ventilation is the intentional air supply to a closed space from the outside, which is essential for the sake of a comfortable environment and the health of human beings. In recent, with the wide spread of renewable energy, much attention has been paid to the natural ventilation. The natural ventilator is classified into a fixed type, a venturi type and a wind turbine type. In this study, the ventilation rates of the fixed type ventilator were experimentally investigated by changing the wind velocity. Additionally, the condition of a backflow was also examined. According to the experimental results, the ventilated air flow strongly depended on the outside wind velocity and also on the intake opening area. In the reverse flow test, it was confirmed that the reverse flow into the ventilator occurred if the wind velocity was under a certain threshold value. Furthermore, the reverse flow phenomenon was more severe when an obstacle is located in the downstream of a ventilator.