A motion planning method for robot groups is presented in this paper. The method aims at considering the tasks of the robots while determining motion of the group. The concept of “cohesion”is proposed to express characteristics of tasks of groups. A motion planning method including“cohesion”is presented. The method is implemented in Virtual Impedance Method, which was proposed by authors. Each robot has two kinds of impedance mode between another robot : one is connecting model, in which the two robots keep near distance each other. The other is independent model, in which each robot moves individually and independently. Each robot decide the mode with the robots of the same group by comparing “cohesion” given to the group and index of danger calculated by environmental situation. Effectiveness of the method is verified by simulation results.