PURPOSE: To enhance measuring accuracy, by setting a condition before a sample solution and a calibration liquid reach an ion electrode region to the almost same level.

CONSTITUTION: A feed line 4 holding plural sets of reaction tubes 2, 3 and moving through a thermostatic tank 1 is provided and a calibrating solution diluted is emitted in each reaction tube 2 for the calibrating solution from a calibrating solution emitting nozzle 5 and a sample solution is emitted in each reaction tube 3 for the sample solution from a sample solution emitting nozzle 6 and, further, a diluent is emitted to the reaction tube 3 from diluent emitting nozzle 7 to dilute the sample solution to predetermined dilution. When the temps. of the solution in the reaction tubes 2, 3 become almost same to the temp. of the fluid in the thermostatic tank 1, the reaction tubes 2, 3 are moved to the arranging position of an ion selective electrode 8, and the calibrating solution and the sample solution are introduced into the electrode region 8 in no special sequence using a common suction pipe 8b and, in a state heat- insulated by the communicating with the thermostatic tank 1 and held to temp. almost equal to the temp. of the fluid in the tank 1, electromotive forces relating to two liquid systems of the calibrating solution and the sample solution are respectively measured.