Provided are a device for evaluating a degree of degradation of a lubricating oil, a method of evaluating a degree of degradation of a lubricating oil, and an on-line lubricating oil management device, which are capable of measuring the degree of degradation of the lubricating oil in a simple and stable manner. The device for evaluating a degree of degradation of a lubricating oil, the method of evaluating a degree of degradation of a lubricating oil, and the on-line lubricating oil management device using the device and the method use a pH-ISFET, and make a determination as to a state of degradation of the lubricating oil through measurement of a change in hydrogen concentration of the lubricating oil by using, as an output, a change in current flowing between a drain and a source in a case where a constant voltage is applied between the drain and the source or a change in voltage between the drain and the source in a case where a constant current is caused to flow between the drain and the source.