We developed a method for estimating effective flow areas not only in the external wall of a house but also in the internal walls between rooms using only one type of tracer gas. This method is based upon the following hypotheses: (1) the temperature and tracer-gas concentration in the room is homogenous, (2) the air in the room is in a windless state, (3) there is no error in the measurement of tracer-gas concentration. However, we know that in reality, these hypotheses are not entirely true. To check the effects of discrepancies between the hypotheses and actual conditions on the estimated results, we set up a miniature model experiment. As a result of this experiment, it was clarified that the present estimation method yields a reasonable estimate of the discharge coefficient even when there is a certain amount of discrepancies between the hypotheses and actual conditions.