An oil concentration measurement system for measuring a concentration of the oil component in a measurement-target liquid in which an oil component is mixed, where the measurement-target liquid contains, as a principal component, a molecule having no multi-valence bond of carbon atoms, while the oil component contains a molecule having a multi-valence bond of carbon atoms, the system comprising: a) an absorbance measurement means (104, 105) for measuring an absorbance spectrum for the measurement-target liquid within a predetermined waveband located between 270 and 400 nm where an absorption of light is observed; b) an oil concentration determining means for low concentrations for determining a concentration value of the oil component based on the absorbance at a predetermined wavelength within the predetermined waveband as well as based on a first calibration curve showing a relationship between the concentration of the oil component and the absorbance at the predetermined wavelength; c) an oil concentration determining means for high concentrations for determining a concentration value of the oil component based on a wavelength at which the absorbance has a predetermined absorbance value in the absorbance spectrum within the predetermined waveband as well as based on a second calibration curve which is created by applying an approximate function to a relationship between the concentration of the oil component and the wavelength at which the absorbance has the predetermined absorbance value; and d) an oil concentration determination method selection means for selecting, based on a predetermined criterion, either the oil concentration determining means for low concentrations or the oil concentration determining means for high concentrations as the means for determining the concentration value of the oil component, wherein the absorbance at the predetermined wavelength or the kind of measurement-target liquid to be monitored is used as the predetermined criterion. a) an absorbance measurement means (104, 105) for measuring an absorbance spectrum for the measurement-target liquid within a predetermined waveband located between 270 and 400 nm where an absorption of light is observed; b) an oil concentration determining means for low concentrations for determining a concentration value of the oil component based on the absorbance at a predetermined wavelength within the predetermined waveband as well as based on a first calibration curve showing a relationship between the concentration of the oil component and the absorbance at the predetermined wavelength; c) an oil concentration determining means for high concentrations for determining a concentration value of the oil component based on a wavelength at which the absorbance has a predetermined absorbance value in the absorbance spectrum within the predetermined waveband as well as based on a second calibration curve which is created by applying an approximate function to a relationship between the concentration of the oil component and the wavelength at which the absorbance has the predetermined absorbance value; and d) an oil concentration determination method selection means for selecting, based on a predetermined criterion, either the oil concentration determining means for low concentrations or the oil concentration determining means for high concentrations as the means for determining the concentration value of the oil component, wherein the absorbance at the predetermined wavelength or the kind of measurement-target liquid to be monitored is used as the predetermined criterion.