To provide a penetrating method for an underground cutoff plate for smoothly penetrating a succeeding cutoff plate into an existing cutoff plate, in a method for constructing an underground cutoff wall by penetrating the succeeding cutoff plate into the existing cutoff plate penetrated in an underground vertical direction, engaging a male engagement part and a female engagement part adjacent each other.

In a method for constructing an underground cutoff wall by providing each one of a male engagement part and a female engagement part on both side parts of a vertical direction of the cutoff plate and penetrating the succeeding cutoff plate mounted attachably/detachably on one side surface of a penetrating frame into the existing cutoff plate penetrated in the underground vertical direction, engaging the male engagement part and the female engagement part adjacent each other, this method is the penetrating method for the underground cutoff plate for penetrating the succeeding cutoff plate into the existing cutoff plate, engaging the succeeding cutoff plate to the existing cutoff plate, by injecting pressure fluid around the male engagement part in an engagement before state of the existing cutoff plate and washing the male engagement part.