PURPOSE: To detect the electric conductivity of a solution containing calcium and the like for a long time by dissolving away an inorganic salt attached to an electrode plate with a cleaning liquid.

CONSTITUTION: An electrode plate 2 is provided in an electrode housing cell 1 with the bottom and both ends open and a cleaning liquid injection capillary 3 is mounted near it. Then, the cell 1 is emmersed into a solution containing inorganic salts such as calcium and magnesium and while the conductivity is detected with a conductivity meter 6, the electric resistance grown with the attaching of the inorganic salts to the electrode plate 2 and lowers the conductivity. Then, a cleaning liquid such as hydrochloric acid is injected with the capillary 3 directly hitting the electrode plate 2 from a cleaning liquid tank 5 to return the conductivity to the normal measured value by dissolving away the inorganic salts. Thus, the conductivity of the solution can be detected continuosuly and effectively for a long time, particularly enabling the monitoring of the concentration of calcium in a lime saturator of a seawater desalination equipment.