To provide a water softening device and its regeneration controlling method capable of detecting the hardness of the supply water to be softened with the lapse of time and controlling regeneration timing based on the detected value.

The water softening device is provided with an inlet water hardness measuring means 10 for measuring the hardness of supply water to the water softener 1, a treated water quantity measuring means 11 for measuring the flow rate of treated water after passing through the water softener 1 and a salt water concentration detecting means 8 for detecting the salt water concentration at regeneration. Further, a hardness overlooking detecting means 12 which measures the hardness of water which passed through the water softener 1 and detects the overlooking of hardness is provided.