There is provided a high-speed and stable robot apparatus realizing stable walking by surely grounding legs having wheels even on an irregular ground surface. The mobile robot includes a body having a control device and an inclination angle detecting device which detects an inclination angle and an angular velocity, and the plurality of legs each having a thigh part and a lower limb part. The thigh part is rotatable around an X-axis, which is a front direction with respect to the body, and around a Y-axis which is a side surface direction with respect to the body. The robot comprises wheels rotatable around the Y-axis and movable supporting parts rotatable around the Y-axis and capable of being grounded on a road surface, which are provided on lower portions of the lower limb parts. The robot switches its operation according to irregularities of the road surface between coaxial two-wheeled inverted pendulum control operation of moving with the wheels without grounding the movable supporting parts while maintaining the inversion and bipedal walking control operation of walking by controlling the grounding of the wheels and the movable supporting parts on the road surface.