A code for authentication is shared between or among a plurality of transmitting devices or a plurality of receiving devices in a transmit/receive system which implements authentication using a code between transmitting and receiving devices. Each transmitting device includes a variable code retaining unit, a control signal transmitting unit, a variable code updating unit, and a unit for transmitting variable code for synchronization. The variable code retaining unit retains a variable code. The control signal transmitting unit transmits a control signal including a variable code. The variable code updating unit updates the variable code after transmission of the control signal. The unit for transmitting variable code for synchronization transmits the updated variable code to other transmitting devices of the plurality of transmitting devices to allow the other transmitting devices to set such code as the latest variable code.