PURPOSE: To prevent the intrusion of concrete by a method in which an interlock part of a trench form is contacted with a sheathing board in a trench cover form, mud water in the trench form is discharged, and the adhesion between the trench form and the sheathing board is raised by a negative pressure entailed.

CONSTITUTION: An interlock part 2 consisting of a trench form part 2a and an interlocking pipe 2b is penetrated into the end of an excavated trench 1a. A sheathing board 3 in which large numbers of iron bars 4 are thrust into an iron plate and fixed is contacted with the trench form part 2a in such a way as to cover the recession of the part 2a in a trench cover form, and mud water in the inside 5 of the trench form is partly discharged through a water-pumping pipe 6. As a result, a negative pressure is applied to the sheathing board 3, the board 3 is pushed toward the inside 5 of the trench form, and thereby the adhesion between them is raised and a water cut-off effect is secured. An iron bar cage 7a is then inserted into the middle of the excavated trench and concrete is placed.