PURPOSE: To obtain an excellent-maintainability, high-accuracy deciding device which is neither affected by water and air bubbles nor requires a reagent, etc., by observing continuously a secular change in current when an alternating voltage is applied to lubricating oil.

CONSTITUTION: A measurement block 3 is provided with a couple of plane electrodes 5 and 6 made of metal such as titanium across an insulating spacer 4 of "Teflon". Then, the alternating voltage is applied from a voltage source 8 to the electrodes 5 and 6 and the current flowing between both electrodes is detected by an ammeter 10. The frequency of the voltage source 8 is preferably 0.1W 10Hz. Output electric power decreases gradually from the start of use (point a) and increases again after a minimum current point (b). For the purpose, an arithmetic processing part 12 detects the minimum current point (b) and the increase rate after that or a relative value to the start point (a) of use is set, so that the inspection and replacement points of the lubricating oil are known. Further, a temperature measuring element 7 provided to the block 3 measures the temperature of the lubricating oil and the arithmetic processing part 12 corrects the output current to improve the reliability of the measurement.