PURPOSE: To obtain the device with high accuracy having excellent maintainability that has no effects of a moisture and bubble and requires no reagent by observing continuously the change in the lapse of time of the potential difference caused between two sorts of electrodes which are brought into contact with a lubrication oil.

CONSTITUTION: A comparative electrode 3 consisting of the material which is difficult to suffer from the potential change, for instance, a platinum due to the additive, etc. in a lubrication oil, and the metallic electrode 4 consisting of the material which is easy to suffer from the potential change due to an additive, etc., for instance, a lead, are dipped into the lubrication oil 2 of the inside of an oil tank 1 and the potential difference EM appearing between both electrodes is measured by a potentiometer 5. The potential difference EM is determined mainly by the additive in a fresh lubrication oil but with the use of the lubrication oil the potential difference by an oxidizing product becomes dominant. Therefore the changing quantity ΔEM which is from the potential difference EM0 in a fresh lubrication oil is inputted in advance into an arithmetic process part 6 and is compared with the potential difference EM to be measured and if both differences become more than the prescribed it is outputted to a display part 11. By the way a temp. correction is performed by the output of a temp. measuring element 7. Also when a voltage is applied from a voltage source 9, the oxidizing product adhered to the electrode 4 is soluted and the electrode 4 can be activated again.