PURPOSE: To know the proper replenishing period of a calcium agent to replenish a proper amt. of the calcium agent by providing a replenishing warning means detecting the ion data in supplied raw water before and after the supplied raw water is changed over to a bypass water passage and issuing a warning on the basis of the detection results.

CONSTITUTION: When a signal turning a relay 50 OFF is transmitted from a control part 53, the applied voltages of operation parts A, B become zero and the tap water of a water supply pipe 4 passes through a bypass water passage 43 and further passes through the conductivity sensor 54 of an ion data detection tank 45 and supplied to an electrolytic cell 2. The conductivity data corresponding to the calcium agent in supplied water is measured by the conductivity sensor 54 to be converted to a digital value by an A/D converter and this value is stored in the control part 53 as calcium measured data at the OFF-time. This data is compared with the calcium measured data stored when the relay 50 is turned ON and, when it is judged that there is no significant difference, an LED 46 for the replenishing warning of the calcium agent is allowed to light. By this constitution, the replenishing period of the calcium agent can be accurately known.