In recent years, research and development of human assist robots have been actively carried out in order to support the independent lives of persons with impaired motor function. Although assisting the user's activity according to the user's motion intention is important for the human assist robots, it is difficult to objectively determine in real time whether the assisted motion by the robot is exactly same as the intended motion by the user. In this paper, a method to objectively detect a user's sense of assist mismatch using EEG signals is investigated. Experiments were conducted to analyze the user's brain activities during the ankle joint movement with the assist of intended and unintended motion. The experimental results show that there are differences in the theta wave activities around the central area of the head and alpha wave activities around the central sulcus when assist mismatch occurs. These results suggest that a user's sense of mismatch can be detected with the proposed method.