An alarm system comprising a plurality of alarms that form a multihop network extending from a relay device, wherein the plurality of alarms include a first alarm (600n+1), a second alarm (600n+2), and a third alarm (600n+3), the third alarm (600n+3) is capable of communicating with the first alarm (600n+1) and the second alarm (600n+2), the third alarm (600n+3) derives a first cost for a first relay route for communicating with the relay device via the first alarm (600n+1), by exchanging information on quality of link with the first alarm (600n+1), and derives a second cost for a second relay route for communicating with the relay device via the second alarm (600n+2), by exchanging information on quality of link with the second alarm (600n+2), the third alarm (600n+3) selects the first relay route when the first cost is smaller than the second cost, when power consumption in the first alarm (600n+1) grows larger than a threshold value, the first alarm (600n+1) transmits a notification indicating an increase in power consumption to the third alarm (600n+3), and wherein the third alarm (600n+3) derives the first cost based on a first indicator (Ka×Max) determined by the information on quality of link with the first alarm (600n+1) and a second indicator (Kb×C) determined by power consumption in the first alarm, and wherein the third alarm (600n+3) derives the second cost based on a first indicator (Ka×Max) determined by the information on quality of link with the second alarm (600n+2) and a second indicator (Kb×C) determined by power consumption in the second alarm (600n+2), when the third alarm (600n+3) receives the notification from the first alarm (600n+1), the third alarm (600n+3) increases the second indicator in the first cost. the plurality of alarms include a first alarm (600n+1), a second alarm (600n+2), and a third alarm (600n+3), the third alarm (600n+3) is capable of communicating with the first alarm (600n+1) and the second alarm (600n+2), the third alarm (600n+3) derives a first cost for a first relay route for communicating with the relay device via the first alarm (600n+1), by exchanging information on quality of link with the first alarm (600n+1), and derives a second cost for a second relay route for communicating with the relay device via the second alarm (600n+2), by exchanging information on quality of link with the second alarm (600n+2), the third alarm (600n+3) selects the first relay route when the first cost is smaller than the second cost, when power consumption in the first alarm (600n+1) grows larger than a threshold value, the first alarm (600n+1) transmits a notification indicating an increase in power consumption to the third alarm (600n+3), and wherein the third alarm (600n+3) derives the first cost based on a first indicator (Ka×Max) determined by the information on quality of link with the first alarm (600n+1) and a second indicator (Kb×C) determined by power consumption in the first alarm, and wherein the third alarm (600n+3) derives the second cost based on a first indicator (Ka×Max) determined by the information on quality of link with the second alarm (600n+2) and a second indicator (Kb×C) determined by power consumption in the second alarm (600n+2), when the third alarm (600n+3) receives the notification from the first alarm (600n+1), the third alarm (600n+3) increases the second indicator in the first cost.