[en] This paper describes an electrostatic integrating radon monitor designed for the environmental radon monitoring and longterm measurements of mean radon concentrations in outdoor and indoor air. The position of the collecting electrode within the monitor was determined based on the calculation of the internal electric field. The radon exchange rate between the monitor and the outside air through the filter was 0.75 h^-1. The exchange rate can make the radon concentration inside the monitor to follow thoroughly the outside concentration. Since the electrostatic collection of RaA⁺ (²¹⁸Po⁺) atoms depends on the humidity of the air, the inside of the monitor was dehumidified with a diphosphorus pentaoxide (P₂O₅) drying agent which is powerful and dose not absorb radon gas. From the relationship between track density and radon exposure, the calibration factor was derived to be 0.52 ± 0.002 tracks cm^-2 (Bq m^-3 h)^-1. The detection limit of mean radon level is 1.2 Bq m^-3 for an exposure time fo 2 months. The mean radon concentrations in various environments were measured through the year using the monitors this developed. The annual mean outdoor radon level in the Tokai districts was 7.0 Bq m^-3. The mean radon concentrations was found to vary from 3.5 to 11.7 Bq m^-3 depending upon the geographical conditions even in this relatively small region. The annual indoor radon concentrations at Nagoya and Sapporo ranged from 6.4 to 11.9 Bq m^-3 and from 15.5 to 121.1 Bq m^-3, respectively, with the type of building material and the ventilation rate. The mean radon concentrations in tightly built houses selected at Sapporo are about 10 times as high as those in drafty houses at Nagoya. (author)