The crucial role in low background experiments plays the suppression of background. One of the most important sources of radioactivity is the radon gas and its daughter products. To be able to measure low radon activities at level of ~mBq/m3 in reasonable time of measurement ultra-sensitive radon detector was developed. The detection is based on electrostatic collection of radon progenies (214Po, 218Po) on Si PIN diode. The volume of radon detector is 50 l and its sensitivity is 10 mBq/m3.