Plastic and small columnar glass cup 222Rn monitoring dosimeters and CR-39 detectors were used. The dependence of indoor radon level on normally used water in toilet in dwelling was surveyed.The ratio of radon level in various monitoring sites is in water of a water tank : below a lid of the tank : on the lid : in bedroom : in balcony = 300 : 5 : 1 : 0.4-0.8 : 0.2-0.3. The effect of normally used water in a toilet of dwelling on indoor radon level is very small if the water tank with a cover is used.