PURPOSE: To estimate the content of water in oil without solvent and reagent and without comparing oil and skill.

CONSTITUTION: Oil to be measured, is inputted into a sealed container 1, whose inner volume is constant. The sealed container 1 is heated with a heater 3. The temperature in the sealed container is made to be the temperature higher than the saturation temperature of vapor corresponding to the pressure in the sealed container 1. Thus, the total amount of the water content in the oil to be measured is evaporated and becomes the overheated vapor. The total pressure P in the sealed container 1 at this time is measured. The partial pressure P1 of the overheated vapor is obtained based on the total pressure P, and the content of the water in the oil is estimated.