The occurrence of perchlorate in water purification plants in the Tone River Basin was investigated. The sampling of raw, processed and treated water at the plants was conducted between September and October 2006. Perchlorate concentrations in raw surface water taken from the upper, middle and lower Tone River Basin were 0.09-0.82, 0.15-15 and 0.15-2.7 μg · l^-1, respectively. Perchlorate concentrations in the upper Tone River Basin were low because the intake points were upstream of the discharge points of perchlorate. Perchlorate concentrations in the lower Tone River Basin were lower than those in the middle Tone River Basin. However, many of the perchlorate concentrations in the lower Tone River Basin were higher than those of the upstream of the discharge points of perchlorate; thus, it was found that perchlorate contamination of the surface water of the Tone River Basin was widespread. Perchlorate concentrations in raw ground water were < 0.05-40 μg · l^-1, and were high at some water purification plants. The shorter the distance between a well and the Tone River, the higher the pechlorate concentration in the well. Perchlorate concentrations in treated surface water of the upper, middle and lower Tone River Basin were 0.12-0.86, 0.22-14 and 0.23-1.8 μg · l^-1, respectively. Perchlorate concentrations in treated ground water were 0.08-24 μg · l^-1. It was shown, in this study, that perchlorate concentrations in treated water were lower than 24.5 μg · l^-1, the Drinking Water Equivalent Level (DWEL) of perchlorate defined by the United States Environmental Protection Agency (USEPA). From the comparison of perchlorate concentrations in raw and treated water at the same water purification plants, it was confirmed that perchlorate was not removed by either conventional or ozone/biological activated carbon (BAC) processes.