A method of monitoring deterioration of lubricating oil and a device therefore capable of judging deterioration of the oil with high accuracy suppressing measurement error due to mixing of foreign matter in the oil. Correlation between relative permittivity and TBN (total base number) of the lubricating oil is obtained beforehand (step S1), electric current and voltage are measured (step S2) when high-frequency voltage is applied between a pair of electrodes located in an oil path in a gas engine, permittivity of the oil is detected (step S3) based on the measurement of current and voltage, TBN of the oil is obtained (step S4) from the correlation between relative permittivity and TBN, and deterioration of the oil is judged by the obtained value of TBN.