Under the influence of biological hazards including COVID-19, it is required sufficient ventilation to decrease the infection risk in the indoor area. In particular, the natural ventilation with window opening is recommended in rooms with inadequate ventilation. However, the ventilation rate, energy loss, and indoor thermal environment with window opening in air-conditioned room varies hourly with given environment. In addition, opening windows in winter causes serious problems such as deterioration of the indoor thermal environment and reduction of absolute humidity. The purpose of this study is to evaluate the changes of ventilation rate, energy loss, and indoor thermal environment with window opening. To achieve this purpose, actual measurements in a residential house during the winter were performed. In the actual measurement, fluctuations in ventilation rate, indoor temperature and humidity distribution, and electric power usage of air-conditioner with the window opening area were measured. In addition, the effect of opening windows on the human body was examined using a thermal manikin. As a result, when a window with a height of 2 m was opened 0.02 m, a ventilation rate of 56.6 m3/h was obtained. In addition, the opening of the windows caused cold draft and deterioration of the indoor thermal environment.