In situ monitoring of a degradation level of refined oil during use is provided. An excitation light irradiation unit varies intensity of excitation light to irradiate the excitation light onto the refined oil with a resulting fluorescence. A fluorescence intensity detection unit detects the intensity of fluorescence generated by the irradiation of the excitation light, and a time lag characteristic calculation unit calculates a time lag characteristic of a fluorescence intensity variation with respect to the intensity variation of the excitation light. A degradation index value acquisition unit provides to a predetermined correlation between a degradation index value indicating a degradation level of the refined oil and the time lag characteristic is used to acquire a degradation index value from a time lag characteristic calculated in the time lag characteristic calculation unit to determine the current degradation status of the oil.