PURPOSE: To enhance the quality of fuel oil, by a method wherein the fuel oil, especially, the fuel oil used in a diesel engine is allowed to pass through a purifying action process due to a purifying apparatus such as a decanter type centrifugal separator.

CONSTITUTION: An improved decanter type centrifugal separator 11 is provided at least behind a heater 5 in order to purify fuel oil and, further, a centrifugal filter is arranged behind said separator 11. By this constitution, continuous purifying treatment operation is applied not only to fuel oil having viscosity of about 180 centistokes (50°C) or less but also to low quality fuel oil having viscosity of about 450 centistokes (50°C) over a wide range. Because no disc type centrifugal separator is used, there is effect in the aspect of maintenance or labor saving.