This paper describes an approach of vision-based external force estimation for mobile robots. The force related motion control such as force control, compliant control, impedance control and so on, requires the force sensing property. And the visual information is one of important resources for recognizing environments in mobile systems. In the proposed approach, external force and robot velocity are estimated only from visual information. Therefore, any internal sensors, such as force/torque sensor, encoder and tachometer, are not required for force sensing. The robot motion is estimated from optical flow field and disparity vector by stereo vision system, furthermore external force can be calculated by reaction force observer. The vision-based approach has a remarkable potential function, that is virtual external force affected by remote object motion in visual scene, can be generated intentionally. The validity of the proposed approach is verified by several experimental results.