Hydrogen has a high diffusion rate and a wide range of combustion, making it essential to take safety precautions against leakage during storage. We investigate the change in hydrogen concentration in confined spaces due to hydrogen leakage through experimental and numerical methods assuming the worst-case scenario. The vent area consists of 12 %, 24 %, and 36 % of the floor area and is installed in the ceiling of the concrete enclosure. We confirm that the improvement of ventilation performance is insignificant just by increasing the ventilation area above a certain standard during emergency ventilation. Moreover, the improvement of ventilation performance by forced ventilation is more effective as the natural ventilation area is smaller. Still, the effect decreases as the natural ventilation area becomes larger. In conclusion, we confirm that ventilation efficiency is determined by the natural ventilation area rather than forced ventilation.