In order to investigate various problems of actual conditions for hybrid air-conditioning system, a questionnaire survey about satisfaction of office environment and understanding of the system, was conducted for office workers of an existing office building with a hybrid air-conditioning system. Field measurement of various indoor environment was also performed simultaneously. According to the result of field measurement, indoor thermal environment cased by natural ventilation was more

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... orm than that of mechanical ventilation, however indoor CO 2 concentration was decreased about 70ppm on average by natural ventilation. On the other hand, according to the result of the questionnaire survey, the rate of the workers who understood the hybrid air-conditioning system was about 40% of all respondents. Therefore, it is necessary to provides accurate information about the system to all workers for sustainable operation.