Abstract: On a shaft fitting member, a plurality of hooked shaft guide pieces of an appropriately resilient plastic material are disposed alternately along the opposite edges of the shaft fitting member confrontingly at staggered positions. In the space formed inside the shaft guide pieces thus confrontingly disposed, there is inserted a plastic shaft having a raised strip with parallel edge faces each adapted to keep sliding contact with the horizontal face of the hooks of the shaft guide pieces on its side. One edge face of said raised strip is provided with one sawtooth projection permitting the shaft to slide only in the push direction and a plurality of trapezoidal projections permitting the shaft to be locked at any of as many selected positions.