A processor (11) of an information transmission device (100) generates pairing information being divided into multiple pieces according to a predetermined data structure and for starting bidirectional communication between a portable terminal and the information transmission device. The processor (11) transmits generated pairing information being divided into multiple pieces. The processor (11) receives a pairing request signal transmitted by the portable terminal that receives the pairing information. The processor (11) performs a pairing operation with the portable terminal in response to the pairing request signal received.
