To provide an ion-concentration measuring system which can prevent a wrong diagnosis due to a positive error caused by disturbing ions which coexist in a sample.

In addition to a base electrode 14a as a chlorine-ion selective electrode a first auxiliary electrode 14a whose selectivity coefficient with respect to lipophilic ions is larger than that of the base electrode 14a, and a second auxiliary electrode 14c whose selectivity coefficient with reference to hydrophilic ions is larger than that of the base electrode 14a, are provided. When a judgment means 20 judges that measured values by the auxiliart electrodes 14b and 14c are larger than a measured value by the base electrode 14a and that their difference exceeds a set value an alarm is generated from an output means 40.