PURPOSE: To find oil concentration by means of the output without using carbon tetrachloride by putting an oil containing sample in a measurement cell after oil content in sample water is extracted into ether, and detecting the transmitted light intensity by means of a light receiving element while radiating the light having a prescribed wavelength.

CONSTITUTION: Oil content contained in sample water is extracted into ether, and a sample is housed in a measurement cell 1 after the oil content is extracted. The cell 1 is arranged on the optical axis of a light source 4, and while controlling a motor 52, for example, an interference filter, whose extractable wavelength is 220nm, is arranged on the optical axis. Switches SW1 and SW2 are connected to the measurement system 10 side. When measured in this condition, the light having a wave length of 220nm is radiated in the cell 1, and the transmitted light intensity is detected by means of a light receiving element 6. A reference light receiving element 7 is arranged in a position opposite to a half mirror 9. Here, an operation is carried out on a ratio of respective detected values of the elements 6 and 7 by means of a ratio operation circuit 11, and the result is displayed on an indicator 13 as information on oil concentration.