To enable quantitative test of peripheral oil type to be calculated from an calibration curve by mixing a light oil sample with a specific rate of dilution agent, alkali solution, and alcohol solution, extracting coumarin in the light oil, exciting ultraviolet ray, and measuring fluorescent intensity.

A test tube 3 in which light oil samples are put is set in a sample chamber 2 of a fluorescence spectrophotometer 1, ultraviolet rays are applied to the samples from a excitation light source chamber 5, and fluorescent intensity of fluorescent light generated from the samples is measured by a fluorescent measuring chamber 6. The sample chamber 2 has an excitation light source window 7 and a fluorescent intensity measuring window 8, and a stirrer 9 is provided at a bottom part of the sample test tube 3. Diluent n-dodecane is diluted with an approx. 4 to 8 volume relevant to an approx. 1 volume of samples, mixed by adding an approx. 4 to 6 volume of alkali solution and an approx. 6 to 10 volume of alcohol solution, shaked, and left statically, and coumarin in the light oil is extracted in solution and hydrolyzed. The extraction solution is excited by ultra-violet rays of approx. 360nm, and mixture rate of the coumarin-added oil in the light oil is calculated from fluorescent intensity of approx. 500nm specific to coumarin based on calibration curve.