This research is concerned with a control performance in an adaptive control method of a mobile manipulator for traveling operations. Several papers of mobile manipulators have presupposed that the traveling terrain is flat. However, the terrain is usually not flat and the shape is unknown. When the mobile manipulator travels on an irregular terrain, the mobile manipulator is subjected to the effect of the disturbance torques derived from the terrain. First, we will propose a mathematical model of a mobile manipulator traveling on the irregular terrain to evaluate the control performance of controllers. The model considers the constraint forces and dynamical influences of pitching motions on an irregular terrain. The dynamical influences of pitching motions of the mobile manipulator give disturbance torques to the mounted manipulator and disturb the accurate operations of the end-effector of a mounted manipulator. Second, an adaptive controller is constructed for the mobile manipulator traveling on the horizontal plane. Third, the validity of the controller is clarified through simulation experiments. Finally, we discuss the control performance of the adaptive controller in the case that the mobile manipulator travels on an unknown irregular terrain.