To make detectable an erroneous wiring by comparing a first AC output being inputted to an A/D converter through wiring with a second AC output being inputted to the A/D converter directly from an AC power supply.

When the arm 12 of a switch 10 is switched to terminal 10a and an AC power supply 11 is applied to respective outputs 11a, 11b, the first AC output 11a applied to wiring 3 is applied from the terminal S1 to the terminal S3 of the exciting winding of a winding type rotary detector 1. The first AC output 11a is applied to winding 3, 4 and taken into an operating unit 10A through an A/D converter 9 under that state. It is subjected to level or phase comparison with a second output 11b and if any one of them mach each other, conduction of the winding 3, 4 and correct connection with the exciting winding can be detected. If the terminal S3 is connected erroneously with S1, it is connected other exciting winding and the first AC output 11a is not delivered from the winding 4. The operating unit 10A judges that the wiring is erroneous through level comparison and the erroneous wiring is displayed.