In this study, the indoor fine dust concentration in an office of the Korea Knowledge Industry Center was measured for about 80 hours when the concentration of atmospheric PM2.5 was very high. The effect of the operation of the air cleaner and the forced ventilation system on the indoor PM2.5 was investigated, and the particle size distribution of the indoor and outdoor particles was analyzed. When forced ventilator and air purifiers were partially used, the indoor PM2.5 concentrations were maintained between 27.7 μg/m3 and 32.9 μg/m3 when the atmospheric PM2.5 was 127.7 μg/m3 to 141.6 μg/m3 during working hours. It is more effective to operate the air purifier without operating the forced ventilation system when the concentration of the PM2.5 is high since the PM2.5 penetrating the installed filter is continuously introduced indoor from the outside.