PURPOSE: To simply and safely decolor an oil at low cost, by blowing ozone at a predetermined temp. in a colored oil such as a mineral oil.

CONSTITUTION: An oil 2a such as mineral oil is fed to the top of a reaction vessel 4 with a pump 2. The oil 2a is heated at a temp. above the melting temp. up to 200°C, and ozone is blown in an amt. of 500mgW100,000mg/l of the oil 2a from an ozonizer 10 (ozone generator) through a diffusing tube 8 into the oil 2a, thereby reacting the ozone with the coloring matter. The pitch 2b thus formed is settled at the bottom of the reaction vessel 4 and is discharged with a pump 6. The exhaust ozone is released from an exhaust ozone treating device 16 into the outside of the system. The oil 2a which has been reacted and decolorized into a colorless state in the reaction vessel 4 is taken out through a with drawal tube 12.