We present a method for controlling mobile manipulators for assist systems. The mobile manipulator should provide good manipulability and stability at any time. Most of mobile assist systems have a heavy mobile base for stability. We propose an approach for motion control of a mobile manipulator to remove unnecessary movement of the mobile base. In our proposed scheme, the mobile base does not move until the manipulator tip position or the mobile base goes over the preferred operating area which compensates the manipulability and the stability. The boundary of the area is the virtual impedance wall consisting of a spring and a damper by impedance control. If the manipulator tip position or the mobile base reaches the boundary, the mobile base starts to move by a repulsive force from the virtual wall and recovers the system conditions. To verify the validity of the proposed scheme, we have carried out experiments employing a graphic simulator and a real robot. The experimental result shows the effectiveness of the proposed scheme.