A method for determining pH of a sample in contact with an element i constituting a pH sensor array by referring to output Voi of the element i defined as follows: Voi = Si × pHi + Gi × Vrm + Ci wherein pHi is pH of the sample in contact with the element i, Vrm is potential of the sample when the sample is in contact with element i, Si and Gi are sensitivity coefficients, and Ci is a constant, comprising: a calibrating step of contacting the sensor array with a standard solution having a known pH and calibrating the sensitivity coefficients Si and Gi and the constant Ci specific to the element i by referring to the output Voi of the element i through the following formula Voi = Si × pHi + Gi × Vrn + Ci; wherein the output Voi of element i is measured by setting the reference electrode at a predetermined potential Vrn; a first measuring step of selecting a first element i1 and a second element i2 located near each other in the sensor array, contacting the sensor array with the sample, and measuring output Voi1 of the first element i1 and output Voi2 of the second element i2; a potential determining step of determining potential Vrm of the sample, on the presumption that pHi1 and pHi2 of the sample contacted with the first element i1 and the second element i2 are equal each other, by referring to the output Voi1 of the first element i1 and the output Voi2 of the second element i2 measured in the first measuring step, and the sensitivity coefficients Si1, Si2, Gi1 and Gi2 and the constants Ci1 and Ci2 regarding the first element i1 and the second element i2 calibrated in the calibrating step; a main measuring step of measuring output Voi of the element i by contacting the sensor array with the sample; and a pH determining step of determining pHi of the sample contacting the element i by referring to the potential Vrm determined in the potential determining step, the output Voi of the element i measured in the main measuring step, and the sensitivity coefficients Si and Gi and the constant Ci calibrated in the calibrating step. a calibrating step of contacting the sensor array with a standard solution having a known pH and calibrating the sensitivity coefficients Si and Gi and the constant Ci specific to the element i by referring to the output Voi of the element i through the following formula Voi = Si × pHi + Gi × Vrn + Ci; wherein the output Voi of element i is measured by setting the reference electrode at a predetermined potential Vrn; a first measuring step of selecting a first element i1 and a second element i2 located near each other in the sensor array, contacting the sensor array with the sample, and measuring output Voi1 of the first element i1 and output Voi2 of the second element i2; a potential determining step of determining potential Vrm of the sample, on the presumption that pHi1 and pHi2 of the sample contacted with the first element i1 and the second element i2 are equal each other, by referring to the output Voi1 of the first element i1 and the output Voi2 of the second element i2 measured in the first measuring step, and the sensitivity coefficients Si1, Si2, Gi1 and Gi2 and the constants Ci1 and Ci2 regarding the first element i1 and the second element i2 calibrated in the calibrating step; a main measuring step of measuring output Voi of the element i by contacting the sensor array with the sample; and a pH determining step of determining pHi of the sample contacting the element i by referring to the potential Vrm determined in the potential determining step, the output Voi of the element i measured in the main measuring step, and the sensitivity coefficients Si and Gi and the constant Ci calibrated in the calibrating step.