A detection system comprising a plurality of detection groups (G0), each of which including: at least one detector configured to detect presence of an event that calls for disaster protection measures and transmit a detection signal based on a detection result relating to the event; and a relay (R1) configured to relay the detection signal received from the at least one detector , the plurality of detection groups (G0) being configured to transmit or receive the detection signal over the plurality of detection groups (G0) via a plurality of the relays (R1), the plurality of the relays (R1) including a first relay (R11) and a second relay (R12) other than the first relay (R11), wherein the first relay (R11) belongs to a first detection group (G1) and the second relay (R12) belongs to a second detection group (G2) being different from the first detection group (G1), and wherein the first relay (R11) is configured to relay the detection signal to the second relay (R12), when receiving the detection signal from the detector of the first detection group (G1) to which the first relay (R11) belongs, of the plurality of detection groups (G0), wherein the second relay (R12) is configured to relay the detection signal to the first relay (R11), when receiving the detection signal from the detector of the second detection group (G2) to which the second relay (R12) belongs, of the plurality of detection groups (G0), wherein the detector and the relay (R1) in each of the plurality of detection groups (G0) are configured to communicate with each other by a corresponding frequency of frequencies, the frequencies being respectively adopted by the plurality of detection groups (G0), and the frequencies being different from each other, and the plurality of the relays (R1) are configured to communicate with each other by a frequency different from any of the frequencies to be adopted by the plurality of detection groups (G0). at least one detector configured to detect presence of an event that calls for disaster protection measures and transmit a detection signal based on a detection result relating to the event; and a relay (R1) configured to relay the detection signal received from the at least one detector , the plurality of detection groups (G0) being configured to transmit or receive the detection signal over the plurality of detection groups (G0) via a plurality of the relays (R1), the plurality of the relays (R1) including a first relay (R11) and a second relay (R12) other than the first relay (R11), wherein the first relay (R11) belongs to a first detection group (G1) and the second relay (R12) belongs to a second detection group (G2) being different from the first detection group (G1), and wherein the first relay (R11) is configured to relay the detection signal to the second relay (R12), when receiving the detection signal from the detector of the first detection group (G1) to which the first relay (R11) belongs, of the plurality of detection groups (G0), wherein the second relay (R12) is configured to relay the detection signal to the first relay (R11), when receiving the detection signal from the detector of the second detection group (G2) to which the second relay (R12) belongs, of the plurality of detection groups (G0), wherein the detector and the relay (R1) in each of the plurality of detection groups (G0) are configured to communicate with each other by a corresponding frequency of frequencies, the frequencies being respectively adopted by the plurality of detection groups (G0), and the frequencies being different from each other, and the plurality of the relays (R1) are configured to communicate with each other by a frequency different from any of the frequencies to be adopted by the plurality of detection groups (G0). <EOS>