To provide an improving method for a soft ground and a construction method for a structure in a soft ground capable of preventing settlement, land slide, uplift, and collapse of ground from occurring in the peripheral ground of the soft ground to be improved.

A soft ground S is excavated by a critical self-support height as deep as about 1.5 m and a plurality of small pontoons 2a are set afloat on an excavated soft ground S and connected to each other by connection members 2b, to form a ship 2c, traveling rails 2e are laid on the ship, a traversing truck 2d is installed on the traveling rail, a soft ground improving device 2f is installed on the traversing truck, the ship 2c is moved by a winch 2m, the soft ground improving device 2f is moved crosswise to improve a beltlike soft ground, therteafter, the ship 2c is advanced to improve the beltlike soft ground, and after it is hardened, a structure is constructed.