To obtain a regenerant for a hard water softening device wherein solubolity is good, and acid washing effect of an ion exchange resin can be expected by mixing a powdered salt with an expandable material expanding by contact with water and forming.

A hard water softening device is equipped with a salt water manufacturing device for regenerating an ion exchange resin. In the salt water manufacturing device, a salt (regenerant) 3 is charged onto a net 2 in a salt water tank 1, and water is put to a water level 4. Then, the salt 3 in a wetting part 3A is dissolved in water to generate salt water 5 of a specific concentration below the net 2. The salt water 5 is taken out via a pipeline 7 to be used for regenerating the ion exchange resin. In this case, the regenerant is manufactured by mixing powered salt with expandable material expanding by contact with water and forming. When the regenerant comes in contact with water, the expandable material expands, and solid salt is made porous to increase a surface area. Therefore, the regenerant is quickly dissolved in water, and the solution can be introduced to saturated solubility in an early stage.