Long term measurements on the airborne particles were made. Investigation on mite-allergen concentration and on the medical symptoms of allergic children in the eight bedrooms equipped with an air-cleaner and occupied by a person including six allergic children were also made. Three of the patients showed significant improvement. The short term particulate concentration decreased rapidly. Although the air-cleaner performance has been expressed so far in the inclination of the concentration decay curves, this method is not suitable when the initial values are low. Thus, authors proposed a new evaluation method which takes into account of the initial and the back-ground concentration.