An oil recovery system, designed to recover oil from oil-contaminated water of a river, lake or sea due to a difference in specific gravity of the liquids. The system includes a 4-stage separation tank 15 having four chambers 51-54. The first chamber 51 includes a hopper-shaped oil floating unit 59 in communication with a piston pump 68 (FIG. 5) and having porous filters 61, 62 therein. The second and third chambers 52, 53 include a plurality of horizontal diaphragms 63 for reducing the processing time for separating oil from water. The system can be continuously operated without being stopped during an oil recovering operation. The oil recovered from discharged pipe 67 is usable so that the system preferably conserves natural resources.