Intention understanding is necessary during the formation of human communication system. Though the linkage between mirror system and intention understanding has been examined by many studies, the evidence of activities of mirror system during the formation of human symbolic communication system, in which no explicit demonstration of actions is involved, are still limited. We recorded the neural activities with electroencephalography during a coordination game with message passing, which involves formation of symbolic communication system. In this experiment, two subjects were separated into two rooms, thus they could not see any movement of communication partners or hear any sound from the partners’ actions. Significant mu rhythm suppression is found over sensorimotor cortex both when the subjects send and receive messages. This preliminary result indicates that intention understanding involved in message interpretation can also induce the activities of mirror system, even without explicit referring to actions.