A system for measuring a foreign material in a liquid, includes a sampling section, vacuum deaerators, foreign material measuring sensors. The sampling section prepares a sample liquid containing various foreign materials having particles of different sizes and numbers and a calibration standard liquid containing particles of known sizes and numbers. The vacuum deaerators deaerate gases mixed in the sample and calibration standard liquids. The sensors detect the foreign materials in the deaerated liquids. These sensors are arranged in front of the deaerators, respectively.