Purpose: The number of apartment houses equipped with heat recovery ventilators has increased. With the heat recovery ventilators in their houses, many residents are trying to change their filters into high-performance filters in order to improve indoor air qualities of their houses. If the residents already know the heat recovery ventilators’ performance of removing ultra-fine particles, it will be possible to enjoy indoor air quality that meets the standard without operating air purifiers. Method: A room of a 85㎡ type apartment unit was selected for the field experiments. Considering that the domestic PM2.5 concentration ‘very bad’ standard is 76μg/㎥ or higher, it was assumed that the indoor ultrafine dust concentration was higher than 76μg/m3 for the experiment. For this purpose, on a day when the external PM2.5 concentration was 100μg/m3 or higher, external air was brought into the room to raise the indoor PM2.5 concentration to 90μg/m3 or higher, and then experiments with various air change per hours(ACHs)were conducted using a heat recovery ventilator. Result: With a case of air flow rate 0.5ACH, the indoor PM2.5concentration was about 40μg/m3 in a steady state condition, when the average PM2.5 concentration in the outside air was 159μg/m3. In fact, it was difficult to reduce it to the legal concentration value of 35μg/m3 in the experiment condition. The indoor PM2.5 concentration 2 hours after the start of the experiment was 48μg/㎥ for the 0.5ACH condition, about 26μg/㎥ for 1.0ACH condition, about 22μ g/㎥ for 1.5ACH condition, and the legal concentration is 35μg/㎥ or less under all conditions except 0.5ACH. recorded.