A method of evaluation a degree of degradation of a lubricating oil, comprising: using a device for evaluating a degree of degradation of a lubricating oil, the device comprising a pH-ISFET which is free from a reference electrode; and measuring a current flowing between a drain and a source of the pH-ISFET with a first circuit wherein a constant voltage is applied between the drain and the source, or measuring a voltage between a drain and a source with a second circuit wherein a constant current is caused to flow between the drain and the source.