To provide a construction method for a closing wall, which enables the closing wall to be easily constructed by eliminating restrictions on the work environment directly below a waterborne structure.

In this construction method, the waterborne structure 11 is arranged on the water, and the closing walls 21a and 21c are constructed in the water directly below the waterborne structure 11 in the direction of crossing the waterborne structure 11. A guide member 23, which is elongated in the direction of crossing the waterborne structure 11 and the end side of which is arranged in such a manner as to protrude to the outside with respect to a position directly below the waterborne structure 11, is installed. On the end side of the guide member 23, a split closing material 25 is charged into the water, so as to be movably supported on the guide member 23. The split closing material 25 supported on the guide member 23 is moved toward the position directly below the waterborne structure 11 along the guide member 23 by a moving means 39. The charging and movement of the split closing material 25 is sequentially repeated for the construction of the closing walls 21a and 21c.