Ozone has become a significant pollutant not only outdoors but also indoors. The ozone emission test was conducted for the sum of forty five air cleaners selected in the online shops and department stores. Negative ion emission test was also conducted for the selected fifteen ones which had the function to generate negative ions. Filter-type and complex type air cleaners emitted ozone less than 0.05 ppm, the standard of indoor ozone emission of air cleaner. Wet type ones emitted no ozone. In the case of ionizer type, 66.7% in the nine ones emitted ozone about 1.4~10 times larger than 0.05ppm. Most of air cleaners used a pin corona discharge emitted large amount of ozone. Those which used carbon fiber emitted little ozone in spite of generating a lot of negative ions. Keywords：Air cleaner, Ozone emission, Ionizer, Negative ion