PURPOSE: To attain the fault diagnosis of a circuit without ringing an alarm device even when a system is in operation by providing a test signal generating means for generating a test signal equivalent to an alarm command signal to each input terminal.

CONSTITUTION: Test signal generating means 21W24 operated at the diagnosis of a device are provided in the same number as that of input terminals 11W14 to generate test signals 121W124 equivalent to alarm command signals 101W104. For example, when a means 21 is operated, the test signal 121 goes to logical 1, and a pulse signal 172 is fed to an AND gate 70 via OR gates 71, 75 and an AND gate 73, and while an input signal 160 to the gate 70 is logical 1, a signal 170 is outputted. Then an FF 76 is set by the signal 170 to light a display lamp 77. Since a signal having a narrow width not operating the alarm device 81 is used for the signal 170, the device 81 is inactivated even when the signal is given thereto. Thus, the circuit is subjected to fault diagnosis by the lighting of the lamp 77 even when the system is in operation without ringing the device 81.