PURPOSE: To prevent leakage of concrete slag by effecting sealing function wherein a cushion piece is elastically deformed so that the piece comes to the same level as a main board surface of a form.

CONSTITUTION: In a slide form, a flange 2 is formed from each side of a main plate 1 of rectangular shape intended for forming a wall of a concrete structure and a recess 3 which is indented stepwise along one side of the plate 1 in the direction of the flange 2. An elastic cushion piece 4 is secured to said recess and the outer surface 4a of the piece 4 is projected a little from the outer surface of the plate 1. And in a dam form construction method, the adjacent flanges 2 are closely joined to each other and a plurality of rectangular forms are vertically stacked in such a manner that the main plate 1 forms one surface to form an assembled body, and ready mixed-concrete is placed successively upwardly from below the concrete structure and said concrete is cured and the assembled body is slid and raised, and these steps are repeated, wherein a slide type form K2 is used as a form which is used by placing the upper side flange on the level of the upper end surface of a lower layer which has been cured and which is one of the forms constituting the assembled body.