Indoor radon concentration in Madura Island region, East Java Province, has been mapped and determined. The obtained data is useful for health policies of radon in Indonesia and can be used as internationally baseline data through UNSCEAR, IAEA, and WHO. The mapping process has been done using a grid system of 20 km x 20 km, where on each grid there are 5-10 passive radon detectors using CR-39 installed in the houses for 3 to 4 months. After the exposure time, the detector CR-39 is etched using a 6,25 N NaOH solution at 70 °C in the oven for 7 hours to grow traces of alpha particles from radon. The imprints are read using a microscope with 400x magnification. The number of trace amounts of alpha particles in CR-39 shows the amount of radon concentration in the house. It was found that radon concentrations on Madura Island were in the range of 15.11 ± 1.07 Bq m − 3 to 126.93 ± 8.98 Bq m − 3 with an average of 58.74 ± 4.15 Bq m − 3. Radon concentration data in the house and GPS location were then entered into MapInfo Software v.10.5 for a radon distribution map. This research obtains the Regional Map of radon in Madura Island as a part of the indoor radon map in Indonesia.