Indoor air quality is the dominant contributor to total personal exposure because most people spend a majority of their time indoors. The purposes of this study were to evaluate the alternative method for improvement of indoor air quality in house after coating titanium dioxide (TiO2) photocatalyst for interior part of the house using nitrogen dioxide (NO2) multiple measurements. To evaluate the alternative method in indoor environment, daily indoor and outdoor NO2 concentrations of an apartment and a detached house were daily measured for consecutive 21 days in winter and summer, respectively. Another daily 21 measurements were carried out after TiO2 coating on wall paper of interior part in houses. All NO2 concentrations were measured by passive filter badges. Indoor air quality models using mass balance are useful tool to quantify the relationship between indoor air pollution levels, ambient concentrations, and explanatory variables. Using a mass balance model and linear regression analysis, penetration factor (ventilation rate divided by sum of ventilation rate and decay rate) and source strength factor (emission rate divided by sum of ventilation rate and decay rate) were calculated. Subsequently, the decay constants were estimated. In this study, magnitude of improvement of indoor air quality could be evaluated by decay constant.