To facilitate the attachment and secure the high fitting strength by attaching a flat-plate closing plate part as covering an end opening of a deck plate on a laying member, elastically abutting an engagement part extended from its edge near the edge of the deck plate, and holding the closing plate part.

An introduction tongue part 18 is inserted into an upper edge inside of a deck plate 1 with a closing tool 4 inclined toward the end opening 3 and the closing tool 4 is pressed against the edge of the deck plate 1 while rotated so as to be erected. Then, the introduction tongue part 18 is pressed down by the upper edge inside of the deck plate 1 so as to be elastically deformed, a clearance is automatically expanded, and the upper edge of the deck plate 1 is guided between an elastic sandwich projection 14 and an engagement edge 12. This constitution can elastically sandwich the upper edge of the deck plate 1 between the engagement part 12 and the elastic sandwich projection 14 so that the attachment face of the closing plate 5 can be held in the attached state abutted against the edge of the deck plate 1.