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Obstacle avoidance is an indispensable function in human-operated mobile robots for providing safe and easy operations. This paper presents a new approach to obstacle avoidance for a human-operated mobile robot in unknown environments. A general type of nonholonomic two-wheeled mobile robot with several distance sensors to detect obstacles is considered. Because the robot cannot move in arbitrary directions due to a nonholonomic constraint, we propose a model reference control approach in which the robot follows a desired trajectory generated by a reference model. The reference model has steering and brake-like functions that are adjusted according to distance sensor information. The stability of the proposed control system is analyzed with a linear model. The effectiveness of the proposed method is confirmed by experiments in which several unskilled operators handle the robot in a corridor-like space.