PURPOSE: To economically produce petroleum regardless of the amounts of petroleum production by using a system in which a platform is provided in a vertically movable manner to legs, and an oil production facility is set on the platform in such a way as to enable the platform to move on the sea.

CONSTITUTION: An elevator 3 is provided to the lower part of a platform in such a way as to enable the platform 1 to vertically move in relation to legs 2, and a manifold 20 is provided to the lower part of the center of the platform 1. Also, on the platform 1, a separator 21, a dehydrolator 22, a heater 23, measuring devices 24, a test maintenance device 25, an utility unit 26, a living quater 27, unloading systems 28 and 29, etc., are set. After the production of petroleum is ended, the elevator 3 is operated to raise the legs 2 from the platform 1 and the platform 1 is floated and moved to another places.