A prototype of the primary standard of radon concentration, to ensure the uniformity of measurements of radon concentrations in dwellings a)nd workplaces, was developed. The source of radon is a solid emanating source of 226Ra. The design of the primary standard allows its use both in the flow mode and in the closed-loop mode. The radon concentration is calculated on the basis of continuous measurements of the emanation coefficient of the 226Ra source. The value of the emanation coefficient is calculated from the ratio of the activities of 226Ra and 214Bi in a solid source, measured using a spectrometer based on a detector of high purity germanium. The total uncertainty of primary standard is about 4,0% (P=0,95). The main contribution to the total uncertainty is made by the error in determining the activity of the emanating source of 226Ra and the sources used to control the calibration of the spectrometer.