We monitored the quantity of airborne microorganisms at 11 points (points A to K) in a dental office on a routine day of use, and tested the bactericidal efficacy of chlorine dioxide (ClO2) gas in the dental operatory after consulting hours. Fallen airborne microorganisms were collected under air-conditioning (AC) in the dental office, and under four conditions in the operatory. Specimens of the microbes were cultivated on nutrient and Sabouraud agar media (NAM and SAM). Many colonies were observed at the entrance hall and on the cabinet in a disinfection room in the NAM and SAM tests, respectively, while no colony was observed at the foot position of the operating table and treatment bed, and above the head position of the operating room in the NAM and SAM tests, respectively. In the bactericidal efficacy test using ClO2 gas, the dental operatory could be kept clean by the use of 4 mg/L-ClO2 gas in addition to the use of an AC with a plasma filter and the HEPA filter.