Mobile radio devices are disposed at the front and rear ends of a mobile body in such a manner that the mobile radio devices can communicate. At least one of fixed radio devices is used as a synchronization base station. The other radio devices including the mobile radio devices are used as synchronization packet relay stations. Each relay station, when powered up, transitions to a synchronization acquiring mode in which a reception of a synchronization packet is awaited. When having received the synchronization packet without intervention of the mobile radio devices, each relay station transitions to an on-ground synchronization maintaining mode, which indicates a high level of synchronization precision, and becomes able to transmit the synchronization packet. When having received the synchronization packet with the intervention of the mobile radio devices, each relay station transitions to an on-vehicle synchronization maintaining mode, which indicates a lower level of synchronization precision than the on-ground synchronization maintaining mode, and becomes able to transmit the synchronization packet. When having received the synchronization packet from another relay station that is in the on-ground synchronization maintaining mode, each relay station that is in the on-vehicle synchronization maintaining mode transitions to the on-ground synchronization maintaining mode.