COMMUNICATION SYSTEM

PROBLEM TO BE SOLVED: To efficiently transmit data over a wide range without holding routing information.SOLUTION: A communication system 101 comprises: a mobile terminal device 10 which transmits a predetermined signal to a predetermined range by wireless; and a plurality of relay devices 20 which relay the predetermined signal transmitted from the mobile terminal device 10. The relay device 20 comprises: receiving means 22 which receives signals in a first period in which the predetermined signal transmitted from the mobile terminal device 10 is periodically received and a second period in which predetermined signals transmitted from other relay devices are periodically received; and transmitting means 23 which transmits the predetermined signal received from the receiving means 22 to the other relay devices 20 when the receiving means 22 receives the predetermined signal transmitted from the mobile terminal device in the first period or when the receiving means 22 receives the predetermined signal transmitted from the other relay devices in the second period.