To provide a method of mooring a large pontoon of about 110 m in length, about 32 m in width and about 14,000 tons in deadweight tonnage for marine-transporting a plurality of structures such as boilers of 500 tons or more per each, to the limited sea surface of a sea route.

A plurality of connecting fittings provided on a front half deck of a hull are connected to the corresponding connecting members of a quay by a plurality of wires 4a, 4b, 4c to fix the front part of a pontoon to the quay, and a spud is slid down inside a guide pipe connected almost vertically to a steel tower 8 provided at the rear part of the pontoon, and the lower end of the spud is made to dash into the sea bottom and fixed. At the time of departure, a wire connected to a hanging hook provided at the upper end of the spud is wound by a winch provided at the pontoon, via a roller provided at the upper end of the guide pipe so as to pull the spud out of the sea bottom.