The purpose of the present invention is to measure pH of a sample with high accuracy in a pH sensor array, without the use of a glass reference electrode. Each time that a sample is measured, the potential Vrm of the sample is identified, and the identified potential Vrm is used to calculate the pH. The outputs Voi1 and Voi2 of a first element and a second element located near one another in a sensor array are represented as follows. Voi1=Si1×pHi1+Gi1×Vrm+Ci1, Voi2=Si2×pHi2+Gi2×Vrm+Ci2. Voi is the output of the element, Si and Gi are sensitivity coefficients, and Ci is a constant, these values having been derived in advance. Here, where the potential Vrm is constant, and the elements located near one another are presumed to be at equal pH (pHi1=pHi2), the potential Vrm is identified by solving a linear equation with two unknowns.