PURPOSE: To prevent the rolling of cylindrical members when bolts and nuts are fastened or loosened and to promote work efficiency when a back-stay is assembled or disassmbled.

CONSTITUTION: A back-stay assembly/disassembly jig 21 is constituted of a jig main body 22 formed in a long and slender plate shape, an interlocking section 23 provided in the basic end side of the jig main body 22 and provided with two interlocking holes 24 interlocked with heads 13A of two adjacent bolts 13, and a ground section 25 coming into contact with the ground G positioned to the front end side of the jig main body 22 and separated from cylindrical members 11 in the direction of the diameter. Accordingly, when the bolts 13 are fastened for connecting the cylindrical members 11, the jig 21 prevents the cylindrical members 11 from rolling by bringing the ground section 25 into contact with the ground G while interlocking the interlocking holes 24 of the interlocking section 23 with two bolts 13 by which the cylindrical member 11 are going to be rolled when the bolts are fastened, and the bolts 13 are interlocked with the interlocking section 23 to prevent the rotation.