PURPOSE: To find the quantity of a desensitizer coating immediately by a method wherein a solution of a coupler varied in the content concentration is brought into contact with the desensitizer-coated port of a pressure-sensitive copying paper to detect the critical color developing concentration which is compared to a quantity of desensitizer coat/critical concentration correlation curve prepared beforehand.

CONSTITUTION: First, a desensitizing ink is printed on the surface to be coated with a color reacting agent of a pressure-sensitive copying paper in a varied quantity of coat and the quantity of coat thereof is measured for various cases beforehand. Then, the critical color developing concentration of a coupler is checked individually for each quantity of coat of said desensitizing ink and then, a correlation curve is prepared plotting values obtained. Immediately after the coating of a desensitizing ink to be measured, a solution of an electron releasing organic coupler prepared beforehand is applied on the coated part sequentially in varied content concentrations. In this manner, the critical color developing concentration of the coupler is detected and compared to the foregoing correlation curve to find the quantity of coat of the desensitizing ink.