A neck injury is improved with a structure having a lifting mechanism part only on one side. While a lifting driving part is provided on a side of one side frame, a coupling driving rod is coupled to a rotation center of a drive gear a driving link is disposed on a side of the other side frame, and the driving link is coupled to the coupling driving rod. A driving force in the lifting driving part operates on the side of one side frame having the drive gear, but on the side of the other side frame, the driving link rotates together with the drive gear via the coupling driving rod, so as to perform a lifting or lowering operation. The shape of a virtual square coupling contact points of the driving link, the side connectors, the coupling driving rod, and the drive gear does not collapse, and thus an external input is dispersed.