To provide a method of laying a large tube in deep water at a reduced cost capable of making a tube easy to lay by adjusting the weight of the tube in deep water with filing fluid of lower density than that of sea water in the tube in deep water to secure designated buoyancy during tube laying work, and providing the tube in deep water with a necessary stable load against movement by securing designated weight under water after laying the tube.

Fresh water 1 is poured into a tube in deep water under a pressurized condition by fluid head difference against sea water during laying work to lay a tube 11 in deep water for acquiring sea water in deep water on a bottom of sea 10, fresh water 1 is discharged from the tube 11 in deep water by rising flow of sea water caused by density difference after laying the tube on the bottom of sea 10. Under water weight of the tube 11 in deep water is reduced to a desired value during tube laying work to improve efficiency of laying work, and under water weight of the tube 11 in deep water is returned to a desired value to secure a stable load against movement.