Air qulaity measurements along with airflow rate measurements of air conditioning system were conducted in an office building occupied by workers with clerical activities. Contaminamts measured were CO2, SPM (suspended particulate matters) NOx and airborne bacteria. Indoor CO2 and SPM levels were found to be quite high, 1,600 ppm (CO2) and 0.25mg/ m3 (SPM), respectively, at their maximum and exceeded Japanese Standards The cause of the high concentration was found to be air leakage in the outdoor air intake duct. The air leakage rate was estimated applying mass-balance model of CO2. Results indicated that the actual outdoor air intake rate was less than halfof that expected. The applicability of the model was examined and it was considered to be practically satisfactory.