Recently, buildings with natural ventilation system have increased from the points of indoor comfort and energy conservation. In this study, field measurement, questionnaire survey and BEMS-data analysis were conducted at a skyscraper office building with hand-operated natural ventilation windows in order to investigate the window performance and actual window operation during a year. According to the measurement survey, the temperature of the air that flowed in through the window has risen remarkably in the cavity of double-skin due to sunlight. From questionnaire survey, we found that the workers feel the necessity of the natural ventilation windows, and prefer manual control to automatic control. From the result of BEMS-data analysis, it was indicated that the window open rate had a high correlation with the outdoor temperature, and reached its peak at around 21 °C. Furthermore, it was shown that a regular enlightenment activity of appropriate window operation led to energy conservation.