There is provided a spreading device, such as a trawl door used for trawl fishing, and more particularly, a trawl door adapted for stable and efficient operation. The door is equipped with flaps mounted below distal panels and connected to driving units, and the flaps are adapted to be controlled remotely, when the spreading device is being towed through the water; the lower distal panel and the upper distal panel have distal surfaces rendering them hollow, wherein the flaps constitute a part of the distal surface.