When data transmission is to be conducted between a master station and a slave station, the master station moves the other slave stations to such a park mode (standby state) that they do not make a communication request. Thus the master station and the slave station can conduct data transmission from and to each other with priority use of a communication network. After completion of the data transmission process, the master station sends each slave station a notification of release of the communication network to return each slave station to the normal operating state.