1 sequentially circularly accesses terminals s each having a specific address and fetches an analog value such as smoke density or a temperature in the form of a current value a voltage value or a digital code the magnitude a change ratio and the like of a fetched signal are calculated by the microprocessor 2 to generate an alarm and or to display an alarm content in fig2 the terminal s connected to the transmission line l is shown having a transmission interface 7 an address setting unit 8 an address comparator 9 a command discriminator 10 a digital input output circuit 11 an alarm determination reference setting unit 12 a comparator 13 and a sensor 20 in the above arrangement when the terminal s is subjected to address polling the command discriminator 10 supplies signals to the digital input output circuit 11 and the alarm determination reference setting unit 12 in accordance with an output from the address comparator 9 at this time if an output from the sensor 20 exceeds a reference value of the alarm determination reference setting unit 12 the comparator 13 generates an output and an abnormality signal is sent to the central processing unit 1 through the digital input output circuit 11 and the interface 7 when the central processing unit 1 determines an abnormality a control signal is sent to an output means 19 in the terminal s through the digital input output circuit 11 if transmission reception of a transmission signal is disabled by malfunction of the central processing unit 1 or short circuiting disconnection of the transmission line l the output from the comparator 13 in the terminal s is sent from the output.