To surely measure an accurate electric conductivity of a circulating cooling water for preventing occurrence of scale due to condensation of the circulating cooling water, by reducing noise related to a circulating cooling water system.

A spacer sensor main body is provided with at least a pair of electric conductivity measuring electrodes 19, 20, and 21 submerged in the circulating cooling water. An electric conductivity measuring circuit 15 is provided to an instrument 16 which measures an electric conductivity of the circulating cooling water among the electric conductivity measuring electrodes 19, 20, and 21. The instrument 16 outputs a cooling water refill signal for refilling a cooling water according to the electric conductivity measured with the electric conductivity measuring circuit 15. A noise reduction electrode 30 is provided which is submerged in the circulating cooling water while it is connected to a reference line 32 of the electric conductivity measuring circuit 15.