To provide an easily handleable water quality regulating device requiring no frequent replacement nor regeneration of ion exchange resin, even if ion exchange capacity of the ion exchange resin is lowered, and capable of keeping water softening efficiency.

Raw water is passed through a water-softening filter medium packed part with the ion exchange resin 18 packed thereinto to soften the raw water. A metal ion sealing agent 33 is provided at the upstream or downstream side of the filter medium packed part, and the metal ion sealing agent 33 is allowed to elute into the softened water having passed through the water-softening filter medium packed part. A dispersant 45 is provided at the upstream or downstream side of the metal ion sealing agent 33, to allow apatite phosphate on the surface of scale to be dispersed.