A rotary mechanism includes an upper seat pan, a lower seat pan and an arc slideway mechanism. The upper and lower seat pans are connected at their sides and can rotate relative to each other. A middle inner slide rail of the arc slideway mechanism, which has a long through-hole in its center and is fixed to the lower seat pan, is positioned between an upper supporting plate and a lower supporting plate, both of which have a long aperture in their respective centers. Fixed parts run through the long through-hole and the long apertures, and fix an upper outer slide rail and a lower outer slide rail. A locking mechanism having portions fixed to the upper seat pan and the lower seat pan, respectively, is arranged to cooperate with the arc slideway mechanism to limit relative rotation of the upper and lower seat pans.