PURPOSE: To improve a fuel oil to have a low viscosity and good combustibility, by rotating the fuel oil at a high speed in equipment by means of a rotor to cause cavitation and separate difficultly processible substance by centrifugal force.

CONSTITUTION: A fuel oil fed into improving equipment 1 through a feed pipe 8 is led between fixed perforated plates 5b and peforated plates 5a revolving at a high speed together with a rotor 3 (4 is a rotating shaft; 13 is a driving motor) for high speed shearing and caviation to disperse a greater portion of difficultly processible substance (sludge, asphaltene, etc.) into fine particles part of them remaining as substances with a large particle size which are difficult to treat). As the fuel oil travels toward the outlet side, a high-gravity liquid portion contg. the hard-to-treat substances is dispersed through an outlet pipe 9 provided in front of a baffle plate 7, while the dispersed low-viscosity liquid portion with a low specific gravity is passed across the center of the baffle plate 7 and is taken out through a processed oil outlet pipe 10.