PURPOSE: To remove water from residual heavy oil securely and effectively, and to make the use of the oil as fuel for diesel engine and a boiler possible, bu equipping a fuel oil system with an atmosphere open type tank wherein the fuel oil is heated to a temperature not lower than the boiling point of water.

CONSTITUTION: The water-separating tank 12 for a fuel oil is equipped between the tank 9 in common use for a recycling cleaning system and the flowmeter 6. The tank 12 consists of a construction wherein the heating oil 14 and the thermometer 15 are attached to the atmosphere open type tank 13, and heavy oil, a fuel oil introduced from the feed pipe 16 is heated to a temperature not lower than the boiling point of water in the tank 13. Water evaporated from heavy oil is discharged from the venting pipe 17 to the outside, and the heavy oil with water evaporated away is made to flow from the taking out pipe 18 through the flowmeter 6 to the mixing column 2.