To provide a bolt easy to fasten/unfasten when used to fix metal rib materials to both ends and the intermediate of a weir plate.

The bolt 5 is used to form a concrete formwork 1. A stick piece 6 to be bitten into the sheathing board 2 is formed on the head of the bolt 5, and the bolt 5 is driven from the side of the sheathing board 2 into insertion holes 4 of the rib material 3 and the sheathing board 2. Thus, the idling of the bolt 5 is prevented to allow easy fastening/unfastening of a nut. After concrete is placed, the rib material 3 as an obstacle portion is easily dismounted in the state that the sheathing board 2 is left, and so the formwork is easily removed. Removal of the bolt 5 is facilitated when replacing the sheathing board 2.