Method for removing oil in cleaning liquid

A cleaning liquid reservoir apparatus equipped with:

• a cleaning liquid reservoir tank as a container for storing cleaning liquid to be fed to an object to be cleaned for performing a cleaning operation,
• a filter equipment provided neighboring to the cleaning liquid reservoir tank,
• a filter element contained in the filter equipment for filtrating cleaning liquid, said filter element being in a cylindrical shape and having an inner cavity within the same, and
• a feeding pump provided in the cleaning liquid reservoir tank for pumping up a cleaning liquid stored in the cleaning liquid reservoir tank, discharging the cleaning liquid through a discharge port of the feeding pump to feed the cleaning liquid to the filter equipment;
• wherein a plurality of annular protrusions are formed on a cylindrical outer circumferential surface of the filter element, and
• the cleaning liquid fed with the feeding pump is caused to flow in a direction parallel to an axis line of the filter element and along the cylindrical outer circumferential surface of the filter element, thus causing the filter element to capture oil in the cleaning liquid.