PURPOSE: To control the concentration of alcohol for improving the wettability of damping water by measuring the refractive index or Brix degree of the damping water of an offset printer by using a refractometer.

CONSTITUTION: The temperature of the damping water in the damping water device tank of the offset printer in operation is measured to adjust the zero point of the refractometer. Then the number of drops of the damping water whose temperature is already measured is set as a measurement sample 4 on the sample refractometer. Light from a light source 1 is incident on a right- angled prism 3 through a mirror 2 and light passed through the liquid 4 to be measured enters a telescope 6 through the right-angled prism 3, thereby measuring the refractive index or Brix degree. Consequently, the measurement is performed over a wide temperature range and temperature compensation for the measured value can be omitted, so when a thermometer and the refractometer are used, a small amount of damping water becomes the measurement sample and the alcohol concentration is easily and accurately measured.