To improve reliability on cutoff performance remarkably by driving steel sheet piles into an improved wall constructed by a soil-improvement method and composed of improved-column rows, in which improved columns are built in a series, in a series.

An improved-column row 10 is constructed as a circle contacting type that the outer circumferences of adjacent improved columns 1... are brought into contact by a soil-improvement method. Steel sheet piles 2... are driven into the improved column row 10 in a series in arrangement, the joint sections 2a... of the steel sheet piles 2 coincide with the centers 1a... of the improved columns 1, before the built improved column row 10 is cured. Finally, the rectangular wall-shaped cutoff wall 3 comprising the improved column row 10 in a series in the length-breadth directions and the steel sheet piles 2 in a series in the similar length-breadth directions along the improved column row 10 is executed. Accordingly, the reliability of cutoff performance can be improved remarkably.