A method of measurement of ²²²Rn concentration in water is described. In the method air is circulated in a closed system consisting of a chamber, water sample bottle, electrostatic precipitator and Si (Li) alpha detector. The concentration of radon is determined by the alpha activity of daughter products (²¹⁸Po, ²¹⁴Po). The lower limit of detection is 0.2 Bq/dm³ in one hour measurement. Calibration of the system is described and results of measurement in several groundwater samples are given.