PURPOSE: To prepare clear oil effectively, by mixing ash-contg. raw material oil with hydrocarbon and keeping the mixt. at high temp. and under high pressure for removal of ash by sedimentation and separation.

CONSTITUTION: An ash-contg. raw material oil 1 and a hydrocarbon solvent 2 selected from among 3W8C paraffinic hydrocarbon, 4W8C mono-olefinic hydrocaron, benzene and alkylbenzene having a boiling point of 140°C or lower, kept at a temp. near the critical temp. of the solvent 2 and under a pressure above the vapor pressure of the solvent at that temp. are introduced into a separating tank 3 and are made to stay for a predetermined residence time for separation into a clear liquid 4 and a residue 11. The clear liquid 4 is led into a flash drum 5 for removal of the solvent 6 in a gaseous state, separation of ash by sedimentation and taking-out of a clear liquid 7 as final product. The residue 11 is taken out as a byproduct residue 14 after a solvent 12 is recovered in a gaseous state in the flash drum 13. The gaseous solvents 6 and 12 are joined together into a gaseous solvent 8 and the solvent 8 is cooled with cooling water 9 in a cooler 10 for reuse as liquid solvent 2.