In human-human communications, we share the common sense and mutually understand. The principal of communication is modeled by an entrainment phenomenon of the brain dynamics. For the robot motion, the principal of the communication is interpreted into the motion synchronization, which is the entrainment phenomenon of the different dynamics. In this paper, we propose a control law for robot motion synchronization to represent the robot communication principle. We design two tap dance robots having different sizes, and realize the motion synchronization by an orbit attractor design method. Experimental result is illustrated and robot-robot communication is realized.