[en] Aerosol size information is very important for risk estimation of radon exposure. Size measurements of aerosol particles attaching radon progeny were made by using the Detati's ELPI (Electrical Low Pressure Impactor), and its attachment function was determined. The impactor, which covers a wide size range from 0.03 to 10 μm in diameter, gives number size distribution in real time by an electrical detection method. Prior to those size measurements, effects of impaction substrates on size classification were studied. Tested substrate materials were stainless steel plate and aluminum foil, and those surfaces were as follows: 1) untreated, 2) silicon grease coated, 3) silicon oil coated, 4) adhesive tape stuck. It was suggested that the use of grease- or oil-coated substrates did not interfere for the ELPI's electrical measurements but prevent particle rebound or redispersion. And it was confirmed that the coating gave no damage on alpha energy spectrum analysis by surface-barrier type solid state detector (SSD). Size distribution data on aerosol number and activity were independently taken for each impactor sample. In the size measurements on activity, the correction of sampling loss at the ELPI's charger unit was made. Comparing between number and activity size distributions, attachment function of radon progeny on aerosol surface was estimated. In the lower submicron range, the attachment strongly depend on the aerosol size. The larger the aerosol become, the less the attachment depend on the aerosol size. The attachment function of the submicron aerosols was in a transient region from surface-area proportion to diameter proportion, as shown in the previous studies. It means that the ELPI is useful for measurements on attachment function of radon progeny on aerosol particles. But the ELPI does not demand monodisperse aerosol as carrier aerosol, and the attachment function can be determined for any aerosols in a chamber or field experiment. (author)