Two-wheel inverted pendulum type of mobile manipulator has no casters and it is expected that quick and robust motion may be realized according to road surface. However the dynamical behavior is so complicated from a viewpoint of motion stability. To address this issue, this paper describes a motion control strategy to realize a stable pushing operation of the two-wheel inverted pendulum type of mobile manipulator. Here the manipulator platform is considered as a passive joint and a stabilization control of the passive joint is achieved by using null space motion of the mobile manipulator. Then workspace observer is also employed to obtain the robust null space motion.The proposed approach is verified by several numerical and experimental results. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.