Windows are widely used for natural ventilation of the various buildings. Especially high level windows as a part of industrial ventilation systems, play a crucial role in natural ventilation. Compared to mechanical ventilation system, natural ventilation has the advantage of lower installation and operating costs. In general, high level windows for industrial buildings have three types; louver type, 45$^{circ}$ open type and 90$^{circ}$ open type. Based on previous studies, it was found that the louver type and 45$^{circ}$ open type are very effective in reducing rainwater penetration, but they did not have enough ventilation efficiencies. Preliminary tests were performed with the various types of windows. It was found that a 90$^{circ}$ open double layer type window was the best among those which tested in our preliminary tests. Simulated rain was used to estimate the amount of rain penetrated through windows and to observe the paths of rain penetration. Various 90$^{circ}$ open windows were tested to find the windows with minimum rain penetration and maximum ventilation efficiency.