A crank element is arranged rotatably between two support elements so as to prevent removal therefrom. The crank element is formed with a pair of generally inverted-L-shaped end portions each having a horizontal first end region and a vertical second end region bent from that first end region. Each of those two end portions is rotatably secured via retainer bush in each of two through-holes formed in the respective two support elements. The two through-holes each has an elongated hole region and a circular hole region. The retainer bush is resiliently deformable and has: a through-bore in which the first end region of crank element is rotatably support; and a groove portion in which the circular hole region of the through-hole is closely engaged.