The Heidelberg Radon Monitor (HRM) that continuously measures the activity concentration of atmospheric 222Radon progeny, which are accumulated on a static filter is described in detail. This includes an introduction into the measurement principle, its hardware realisation, the firmware as well as the spectra evaluation software. Further, the automatic procedure of spectra analysis and transfer of evaluated data to the ICOS Atmospheric Thematic Centre in Near Real Time are presented. Finally, the procedure of manual (monthly) filter change is described in this manual.