Two-wheel wheelchairs have advantages over conventional four-wheel wheelchairs in terms of mobility and ability of going over steps. However, two-wheel wheelchairs are unstable system, and the body of the wheelchairs must be stabilized. In the conventional method, stabilization control was achieved, but the only stability of the body has been considered and the motion of the wheel has not been controlled. In this paper, a robust wheel-velocity control is proposed in order to improve the stability and the operability of the two-wheel wheelchair. We realize both stabilization of the body and velocity tracking of the wheels using sliding mode control with disturbance observer. The validity of the proposed method is confirmed by some experimental results.