To prevent clogging from occurring in an electric deionizer by scales resulting from water-hardening components in a pure water preparation system equipped with a water softener.

The pure water preparation system 1 comprised of a water softener 4, a reverse osmosis membrane device 5 and an electric deionizer 6 each disposed on a water supply line 2 for supplying water to the devices is characterized by comprising a raw-water hardness measurement device 7 connected to the water supply line 2 on the upstream side of the water softener 4, a flow-rate measurement device 8 connected to the water supply line 2 on the downstream side of the water softener 4, and a controller 9 that outputs a signal to the water softener 4 for starting its regeneration when the product of the measured value measured by the raw-water hardness measurement device 7 at predetermined intervals and the cumulative amount of water measured by the flow-rate measurement device 8 having passed through the water softener 4 from the beginning of the water supply has reached the maximum mass of the water-hardening components that can be removed by the water softener 4.