To prevent the breakage of a panel during the carriage or during the connection work by arranging a projecting link inside of the panel body in a freely retractable manner, firmly tightening and fixing the upper and lower panels to each other, and storing the projecting link inside of the panel body.

A panel body 1 of a rectangular flat plate is provided with a received link 2 which is extended in the right-to-left direction on its upper part, a projecting link 4 in its inner part, and a dovetail groove 81 as a recessed connection member 8 extended in the right-to-left direction in an inner surface. The projecting link 4 is threadedly engaged with a cylindrical body 30 to keep a lower rectangular skeleton in a vertically movable manner, and a dovetail tenon shaped rotation locking tool 40 provided on a lower end of the skeleton is turnable. The projecting link 4 on the upper panel 1 is inserted in the recessed link 2 of the lower panel 1, the rotation locking tool 40 is turned, and a dovetail tenon is engaged with the dovetail groove of the recessed link 2 and firmly tightened. A projecting link 85 comprising the dovetail tenon to be fitted to the dovetail groove in the inner surface of the panel 1 is connected to the panels 1, 1 opposite each other using a connection bar 83 provided on each end, and the concrete is placed.