PURPOSE: To efficiently provide highly pure retentate for dewaxing accelerator at a high temperature and under high pressure, by contacting stream of dewaxing oil with an ultra filtration membrane made from polyimide polymer.

CONSTITUTION: Stream containing dewaxing oil and way constituent is heated to by fluid, the fluid is contacted with the ultra filtration membrane made from polyimide polymer preferably under the condition where temperature is about 70-250°C and pressure is 50-200 psi, the retentate which contains the dewaxing accelerator and can be reused in the solvent dewaxing process is produced, and the dewaxing accelerator is recycled.