PURPOSE: To prevent accident due to the abnormality of the lubricating oil by pushing a photocell into an oil tank, taking out the current generated by irradiating the light having a constant luminous intensity to said photocell, and measuring the suspension degree of the lubirating oil to judge its quality.

CONSTITUTION: An attaching hole 9, to which a light source 8 having a constant luminous intensity is attached by an appropirate method, is perforated in a side wall of the oil tank 1 on the opposite side of an attaching hole 2, to which a sensor 5 of the detecting device is attached. A cable 6 which is guided out of the sensor 5 is connected to an amplifier 14. The magnitude of the current or the variation in the current generated in the photocell 4 based on the intensity of the light irradiated from the light source 8 is sent to a monitor 15. When a specified range is exceeded, an appropriate warning device is actuated. In this device, when abnormality occurs, warning is immediately issued, and the accident due to the abnormality of the lubricating oil can be prevented beforehand.