An air-oil and oil-water interface detector composed of bent plastic optical fiber, photodiode, light emitting diode (LED) and five analog electric circuits was developed and the sensor system was applied to measurement of the oil layer thickness of kitchen wastewater in a grease trap (oil separator for industrial kitchen wastewater). The raw output voltage of the developed sensor system depends on the refractive indexes of the air, soybean oil and tap water. When the sensitive part of the sensor passes through the oil layer, the air-oil and oil-water interfaces can be detected by the peak of the first derivative data calculated from the output voltage of the sensor system. The optimal sampling interval of the output voltage and the optimal movement speed of the sensor are between 100 and 400 ms and over 2.56 mm/s, respectively. The detection limit of oil layer thickness is approximately 3 mm at a 100 ms sampling interval and 2.56 mm/s movement speed. To evaluate the developed sensor system, the thickness of the oil layer in the actual wastewater was measured. Good agreement between the conventional analysis value obtained by a hexane extraction method and the calculated value obtained by the developed sensor was observed. The developed plastic optical fiber sensor system is useful to measure the amount of the floating oil (oil layer thickness) in a grease trap installed in the stream of the wastewater discharged from an industrial kitchen, and the sensor materials are very cheap. A sensing system for detecting the interface of the various materials can be constructed because the refractive index of the materials may be detectable by the developed sensor.