of apparently much declined influence of substances remaining undissolved such as protein on the measurement potential level or electromotive force measured by the ion selective electrode unit 1 is amplified at a preamplifier 5 converted into digital signal by a d converter 6 and stored on a memory 8 under the control of a central processing unit cpu 7 the positive potential level of a correction solution in normal state is measured in the positive ion selective electrode unit 1 and stored as the standard positive potential level e o shown in fig3 a so as to remain constant on the memory 8 likewise the negative potential level of the correction solution in normal state is measured in the negative ion selective electrode unit 1 and stored as the standard negative potential level e o shown in fig3 b so as to remain constant on the memory 8 in addition to the standard positive and negative potential levels on the memory 8 positive and negative potential levels of the correction solution e 39 s and e 39 s respectively and positive and negative potential levels of test solution measured at time intervals are stored cpu 7 functions to compare every measured e with e o and every measured.