Because of the need of energy conservation and Business Continuity Planning (BCP), natural ventilation system, which basically does not use non-renewable energy, is attracting academic/practical attention. However, it is difficult to predict the natural ventilation performance even after completion of the building, because it is easily affected by unstable conditions, such as outdoor temperature and wind. The designing and controlling method of natural ventilation system is not yet sufficiently established. The designers are referring to the past cases, and are still designing it by rule of their thumb. In order to establish the designing and controlling method of this system, unclear phenomena has to be revealed, and the knowledge based on actual operation of the building has to be accumulated. The aim of this study is to establish the designing and controlling method of natural ventilation system for non-residential buildings. The performance of natural ventilation apparatus at an actual city hall with natural ventilation system is presented in this paper. The city hall consists of lower floors (1F – 3F), higher floors (4F – 7F) and three solar chimneys. Higher floors, which consist of office areas, are placed on the lower floors, and solar chimneys is placed alongside the higher floors, facing south. The lower floors, which is a large enclosure, consist of office areas and an atrium lobby at the middle. During the mid-season, the air flows into the building through the natural ventilation openings underneath the windows at the office areas, which open and close depending on the outdoor/indoor conditions, while the air flows out from the top of solar chimneys. Air flow rate was chosen to be the index of natural ventilation performance. The measurement was conducted in fall 2017, and the flow rate through the solar chimneys was obtained by three methods, i.e., airflow velocity measurement, tracer gas method, and measurement of pressure difference. As a result, referring the result of velocity measurement was the most reliable way of calculating the air flow rate. The velocity distribution was measured at solar chimneys in spring 2018. Additionally, vertical temperature distribution was measured in the solar chimneys, and velocity distribution measured at the natural ventilation opening of office areas. The measurement at holiday, which applied heating and lighting as heat load, showed a good agreement with the measurement at week day, whose heat load was from the real workers and heat sources, which means the measurement was reliable. Air change rate was approximately 10 times/h, and it was 1.5 times of the performance prediction result at designing phase. It was also shown that it is possible to estimate the air flow rate from the BEMS data if only the method has fixed. Moreover, the result shows that measured vertical temperature distribution inside the solar chimneys can be beneficial for estimating the flow patterns.