[en] The ultimate purpose of this study was to derive the population dose of Japan. Radon daughter, not radon itself, should be considered as a major influential component for the human body in estimating the average individual dose (per caput dose) due to radon family. The measurement of radon daughters at many points is, however, difficult from both technical and financial points of view. Most practically, it is then recommended that a large survey for the radon gas concentration and a measurement of the radon daughter concentration be conducted at the limited number of points. In this respect, a nation-wide survey of the radon gas concentration has been carried out by using 500 passive radon monitors with electrostatic field over more than 1.5 years. Long-term continuous measurements of individual radon daughter (Ra-A, B, C) concentrations and their free component concentrations have also been performed. In calculating the effective dose equivalent, characteristics of Japan and Japanese should be taken into account as parameters. A survey revealed the radon gas concentration to be approximately 10 Bq/m³ indoors and approximately 5 Bq/m³ outdoors. The equilibrium factor was estimated to be approximately 0.6 for outdoor conditions and approximately 0.5 for indoor conditions. The fraction of free components was estimated to be below 0.05. On the basis of the above equilibrium equivalent factors and fractions of free components, as well as the particle size distribution, the dose conversion factor was similar to values reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (1982)-- that is, 8.7 nSv/(Bqh/m³) for indoor conditions and 17 nSv/(Bqh/m³) for outdoor conditions. The occupancy factor was considered to be 0.2 for outdoor and 0.8 for indoor conditions. Combining these parameters is necessary for calculating the national dose caused by radon family. (N.K.)