To surely prevent co-lifting of a buried sheet without providing heavy equipment at a penetration frame, and further to improve the safety of construction and working environment.

A lower end of a buried sheet 16 is engaged with a lower end of a penetration frame 14, and the sheet 16 is strained to one side of the frame 14. In this state, the buried sheet 16 is lifted down into a groove formed in the ground together with the penetration frame 14, and thereafter the engagement of the buried sheet 16 with the lower end of the penetration frame 14 is disengaged, followed by lifting up the penetration frame 14, whereby the buried sheet 16 is left underground. The lower end of the buried sheet 16 is fixed to a sheet settling member 19, and a vertically moving rod for settlement 24, for fixing the depth position of the lower end of the buried sheet 16, is hooked on the member 19, whereby the rod 24 is relatively vertically movable with respect to the penetration frame 14.