Collision avoidance subject to operator's intention is needed for power-assisted omni-directional mobile robots. In order to realize collision avoidance on operator's initiative, the system which makes the operator notice the existence of an obstacle is necessary. In this paper, existence of the obstacle is notified through a change in operation feeling by adjusting viscosity coefficients of the power-assist system. Two methods of adjusting viscosity coefficients proportionally or exponentially according to the distance between the robot and the obstacle are considered. In an operation experiment at a narrow passage, a subject operated a power-assisted omni-directional mobile robot by wearing a blindfold or a tunnel vision experience goggle. From the results, the subject was able to operate the robot without collision. Additionally, the operator was able to operate the robot smoothly by the exponantial viscosity coefficients adjusting method.