PURPOSE: To provide an oil/water separator to automatically drain only a water content.

CONSTITUTION: By passing oil, sucked by an oil circulating pump 1, through the mesh of an oil/water separating filter 4, a water content contained in oil is separated and introdeced to a closed container 7 by using a specific gravity difference between oil and water. By using a level sensor 8 having a float 9 of specific gravity intermediate between specific gravity of oil and that of water and detecting the level of a stored water content, a solenoid valve 10 opens to automatically drain only a water content.