PURPOSE: To measure accurately and continuously the concentration of contained oil regardlessly of the concentration, and to use by incorporating in the manufacturing line of a factory, by measuring the fluorescence from a liquid to be detected in a thin film state formed on the surface of a member.

CONSTITUTION: A roller 12 is rotated in a state that the lower part is immersed in a liquid to be tested, and the upper part is exposed on the liquid surface. At the initial revolution, the lower part of a roller immersed in said liquid is brought out from the said liquid in accordance with the rotation of the roller, and the said liquid attaching to the surface of the roller in a dropwise state is shaken off to form a uniform film of the said liquid on the surface of the roller. An optical system is provided above the roller 12 to irradiate light on the thin film and to measure the oil concentration by receiving the generated fluorescence. The main part of the optical system is composed of a light source 1 positioned obliquely to the left above the roller 12 and a photoelectric sensor 3 provided obliquely to the right above the same roller. If the light such as UV is irradiated from the light source 1, the fluorescence generated from the said liquid formed on the surface of the roller 12 reaches the photo-electric sensor without being scattered and absorbed to make it possible to measure accurately the oil concentration.