To strengthen joining force and facilitate an assembly and disassembly by inserting a connection member having two legs with the same section to grooves on both surfaces of plural sheet members and providing a spiral projection around the groove and the cylindrical body the same as the width of the recessed part of a coupler inserted and engagingly locked to the recessed parts of the legs.

Grooves 11a, 12a, 12b with a U-shaped type section are formed on both surfaces of first/second members 11, 12 and the connection member 13 installed in parallel legs 14, 15 and 16, 17 with the same section shape as the groove in a right angle direction is engagingly locked by the coupler 10. The leg 14 is inserted in the groove of the groove of the back side of the member 11 and the leg 15 is inserted to the groove 11a until the base part 17b is contacted with a lateral groove 11b. The legs 16, 17 are also inserted in the grooves 12b, 12a, horizontally alike. The coupler 10 of a circular body is inserted in the recessed parts 14a-17a pointing to the outside of respective legs, while forming the longitudinal direction of the recessed part of the leg and diameter (m) direction in the right angle and the hole 4 in the outside and rotated by 90°. Then, the width (n) of the projection of the coupler 10 is contacted to the inner walls of grooves 11a, 12a, 12b and the members 11, 12 can be stuck strongly.