Abstract: An oil detecting system for detecting a leakage oil by optical fluorescence measurement means even in the presence of external fluorescent light, for example, from parts and equipments of a vehicle and a plant comprises an irradiation apparatus for irradiating a light including an absorption wavelength of an oil to be detected and exciting molecules of the oil to make the oil fluoresce, a wavelength selection apparatus operatively connected to the irradiation apparatus and adapted to select a fluorescence wavelength of the leakage oil fluoresced by the irradiation apparatus, and an observation apparatus for detecting the fluorescence of the leakage oil and selecting only a period of observing fluorescing of the leakage oil over substantially the same time period that the leakage oil fluoresces light. A processing apparatus may be further provided so as to be operatively connected to the observation apparatus and adapted to process an image or a signal from the observation apparatus.