Purpose / Context - Indoor airborne bioaerosol, which has an outdoor origin, plays an important role in determining the exposure of humans to bioaerosol, since people spend most of their time indoors. However, there are few studies focusing on the indoor bioaerosols that are generated outdoors. Methodology / Approach - In this study, indoor-outdoor size resolved concentrations of airborne fluorescent bioaerosol in an office room were measured continuously for six days (144 h) by using a fluorescent bioaerosol detector. We focused on the particulate matter with an aerodynamic diameter 2.5μm. Results – The ratios of indoor to outdoor particle concentration and those of bioaerosol to aerosol concentrations in various size ranges vary significantly. The measured and predicted concentrations have a linear relationship for the studied size fractions, with R2 of all size fractions larger than 0.83. Key Findings / Implications – Correlations between indoor and outdoor bioaerosol concentrations show significant concentration-attenuation and time-lag over time. Originality - A two-parameter, semi-empirical model was proposed to predict the concentration of indoor bioaerosol with outdoor origin.