PURPOSE: To prevent a burning accident of a switch beforehand by comparing the voltage at a point of connection between the captioned stoppage position setting switch and the resistance which is connected in series to said switch with reference value, and having an abnormal information signal sent out according to the result of comparison.

CONSTITUTION: When this contrivance is applied to a trouble discrimination device of a stoppage position setting switch 5 for a motor 1 which controls driving of an air mix door opening, the point of connection between a resistance 3 connected to a switch 5 consisting of a movable contact piece 5a and fixed contact pieces 5bW5d through diodes 6, 7, and a motor 1 is connected to a switch 8b of a switch circuit 8. Then, a contact point 8 of the switch circuit 8 is connected to an input terminal 2b. Further, the output terminal side of the switch 8b is connected to the nonreversal input side of a comparator 10 and, when the switch circuit 8 is placed in ON state by means of the output of a central operation processing unit 13 during rotation of the motor 1, reference voltage Vl is compared with voltage Q. Then an abnormal information indication device 21 is put in operation according to the result of this comparison.