Abstract: An oil skimmer has a lower hull capable of floating and submerging and capable of discharging sea water through an opening to thicken a layer of oil. An oil drawing device in the form of a water wheel is installed on the deck of the lower hull for conveying an oil layer on the sea surface to a central tank when the lower hull is submerged. A main floater provides a floating force for the deck and has a guide path partly communicating with the central tank. An oil separating/extracting device is supported at a level above the water wheel of the oil drawing device for separating an upper oil layer in the central tank and transferring the oil through the guide path into an oil collecting tank.