PURPOSE: To decide on the deterioration extent of the cooling water of an internal combustin engine easily, by detecting the amount of copper anticorrosives in the cooling liquid as a difference in the value of color and comparing it with a reference color sample chart.

CONSTITUTION: For a test, a solution to be tested is sampled and put in a test tube 3, and a reagent 1 for the precipitation of excess is added thereto. This reagent 1 is put in the solution to be tested in the state of powder, grains or other solid, or solution to react with the copper anticorrosives, producing a colored precipitate. The copper anticorrosives all react with copper ions in the reagent 1 and excessive copper ions remain. The produced precipitate is removed by filter paper and a reagent which produces a luminescent compound by reacting with copper ions of dimethyl sodium dithiocarbamide, etc., is put in the filtered liquid and the luminescent compound is extracted by extraction liquid such as ethyl acetate and compared with the reference color sample chart differing in color density according to the concentration of copper ions, thus deciding on the deterioration extent of the tested liquid.