14 15 16 are all out meanwhile the electric potential of the common terminal 21 is maintained at a level of the power source 30 the transistor 25 is 34 on 34 transistor is 34 off 34 and thus the primary warning lamp 29 also remains inactive on the contrary if there occur malfunctions or abnormalities at any part or object to be monitored e g the one associated with the sensor 1 then the transistor 6 in the switching circuit 9 becomes 34 on 34 to light up the display lamp 10 the resultant current flow through transistor 6 and resistor 8 lowers the potential at the collector of transistor 6 and due to the diode 17 interconnection between said collector and common terminal 21 the voltage level of the common terminal 21 is also lowered so that the transistor 25 is turned 34 off 34 transistor 26 34 on 34 and thus the primary warning lamp 29 is energized to be lit up this mode of warning operation is equally applicable to other sensors such as 2 3 and 4 however such ordinary monitor system described so far has had the problem that in case of failures or malfunctions possibly occurring in any one of the switching circuits due to damages of circuit elements or disconnections of circuit lines etc it can never be possible to.