Water softening system includes water softening tank, neutralization tank, electrolysis tank, and treatment tank. Water softening tank softens raw water containing a hardness component with weakly acidic cation exchange resin. Neutralization tank neutralizes a pH of soft water that has flowed through water softening tank with weakly basic anion exchange resin. Electrolysis tank generates acidic electrolyzed water for regenerating weakly acidic cation exchange resin of water softening tank and alkaline electrolyzed water for regenerating weakly basic anion exchange resin of neutralization tank. Treatment tank mixes the acidic electrolyzed water that has flowed through water softening tank and the alkaline electrolyzed water that has flowed through neutralization tank, and supplies the mixture of the acidic electrolyzed water and the alkaline electrolyzed water to electrolysis tank.