Abstract: A fish cage for cultivating fish comprises a hull shaped vessel composed of outer side platings, inner bottom platings, screen bulkheads and a deck. A device for floating a hull shaped vessel on the water is provided, and openings are formed in the outer side platings and inner bottom platings and covered with nets so as to let sea water flow in and out of the hull shaped vessel. The device for floating the vessel on the water is formed by tanks arranged on the upper side of the outer side platings to hold air. Openings are provided in the bulkheads and covered with nets affixed to the screen bulkheads for letting sea water flow in and out. Openings are also provided in the outer side platings on the bow and on the stern of the vessel for letting sea water flow in and out.