PURPOSE: To purify crude lubricating oil in high purity industrially advantageously and to enable the repeated use of lubricating oil, by blowing an inert gas into a crude lubricating oil containing a low-boiling organic solvent while heating it at a specific temperature.

CONSTITUTION: As a crude lubricating oil containing a low-boiling organic solvent preferably consisting of methylene chloride is heated at a temperature ≥the boiling point of the solvent, ≥ the boiling point or decomposition temperature of the lubricating oil, to make the viscosity of the crude lubricating oil ≥30cst (preferably ≥20cst), an inert gas is blown into the crude lubricating oil in a rate of 2W5 times as much the inert gas as the crude lubricating oil by volume/minute, and the treatment is carried out usually for ≥20min (preferably 1W3hr), to purify the crude lubricating oil.