PURPOSE: To prevent a flushing phenomenon from occurring, by installing a low pressure fuel oil feed pump in a piping system ranging from a fuel oil tank to an air separator as well as installing an overflow piping system installed in parallelism ranging from the air separator to the fuel oil tank.

CONSTITUTION: Inferior fuel oil inside a fuel oil service tank 1 is fed to an air separator 2 by a low pressure fuel oil feed pump 14 by way of a filter 9. And, fuel oil inside the air separator 2 is pressurized by a fuel oil booster pump 4 and then fed to a main engine built-in fuel oil pump 7 of a main engine 6 by way of a flow meter 3 and a fuel oil heater 5. Surplus fuel oil in the main engine 6 is put back to the air separator 2 from the said pump 7 via an accumulator 13, a pressure regulating valve 10 and a flow meter 23. Likewise, the surplus fuel oil is put back to the saod tank 1 from the air separator 2 via an orifice 8 through an overflow piping system 28. With this constitution, such a flushing phenomenon as being attributabale to evaporation of moisture in the inferior fuel oil is thus prevented from occurring.