An apparatus for measuring oil oxidation is capable of being mounted to mechanical devices for detecting power of fluorescent light reflected from oil in real time as an indication of the oil oxidation. A plurality of first optical fibers are connected to a light-emitting member for introducing the light into oil at different incidence points. A plurality of light-reflecting mirrors are immersed in the oil and disposed opposite to the incidence points. The light-reflecting mirrors are located at different distances from the incidence points. A plurality of second optical fibers are connected to the incidence points, respectively. A plurality of light-receiving members are connected to the second optical fibers for receiving the light and outputting the signals corresponding to the received light. A control portion calculates a fluorescence quantum yield and a light absorption coefficient of the oil based upon the signals from the light-receiving members and compares them with the reference values pre-stored in the control portion to determine the oil oxidation.