Abstract: An apparatus for detecting the deterioration of an engine oil including a ceramic heater radiating infrared light including infrared radiation having a wavelength of 6.1 micron meters which is equal to the specific infrared absorption peak of ester of nitric acid contained in the engine oil. The infrared light is made incident upon a photodetector via a band pass filter having a center wavelength of 6.1 micron meters to detect an amount of the ester of nitric acid contained in the engine oil. It has been experimentally confirmed that an amount of the ester of nitric acid is proportional to the total acid value which is a measure of the deterioration of the engine oil, so that by suitably processing an output signal of the photodetector, it is possible to detect the deterioration of the engine oil. The thus detected deterioration may be displayed on a display device provided on a front pedal of a car.