PURPOSE: To prevent malfunction and also to reduce power consumption by providing more than two constantly-supervisory and stand-by fire detectors in the same precaution areas and by driving a receiver responding only to the logical sum among signals from those detectors.

CONSTITUTION: In the same precaution area, superviosry detector 5 available for constant operation and stand-by detector 8 applied with a voltage via switching circuit 9 are both provided and the receiver is driven responding only to the logical sum between signals from detectors 5 and 8. If a fire occurs, switching circuit 9 turns ON through the operation of detector 5 and detector 8 starts operating. Logic circuit 12 operates by signals from the both, thereby sending an alarm signal to the receiver. Only detector 5 among detectors consumes power constantly, so that the whole power consumption can be reduced. In addition, even if detector 5 would malfunction due to momentary power variation, detector 8 is never influenced by power variation at the same time, so that an alarm signal will never be generated by mistake.