PURPOSE: To simply and easily fix a partition wall, by holding a fish joint plate to the flange part of the partition wall and forming the same into a shape to which the function preventing the rotation of a bolt head is applied.

CONSTITUTION: The inner periphery 6 of the head 4 of the bolt 3 inserted in the hole 2 of a fish joint plate 12 is formed into a hexagonal shape so that said bolt 3 is not rotated when fastened by a nut 5. A partition wall 11 constituted of a unit panel 11A in the same way as a bottom wall 10 is equipped with the flange part 12 of the unit panel 11a at the lowermost position thereof. The flange part 12 is placed on the bottom wall and fastening bolt piercing holes 13, 14 are provided to both of them by drilling. The fish joint plate 1 is placed on the flange part 12 along the longitudinal direction of said flange part 12. The hole 2 and bolt piercing hole 7 of the fish joint plate 1 are allowed to coincide with the bolt piercing holes 13, 14 and the bolt 3 having the head 4 corresponding to the hole 2 of the fish joint plate 1 is pierced from the inside of a tank to the outside thereof and the nut 5 is screwed on the leading end of the protruded bolt 3 on the outside of the bottom wall 10 to fasten the bolt 3.