PURPOSE: To prevent the run-away of a robot and an NC device by detecting the disconnection of a cable from an output available from converting a current flowing in the primary coil of a revolver into a voltage, an output voltage from a secondary coil and the added result of a reference voltage.

CONSTITUTION: Primary current detecting circuits 20 and 24 connected between cables 6 and 7 and cables 8 and 9 connecting terminals S1 and S2 and those S3 and S4 of the primary coil 3 of the resolver 1 and a resolver controller 12 detect input currents Isinωt and Icosωt, and input them to shaping circuits 23 and 27 through amplifiers 21 and 25. A secondary detecting circuit 28 connected between terminals O1 and O2 of the secondary coil 4 of the resolver 1 detects the output voltage of the resolver 1, and inputs it to a shaping circuit 30. Then an adder circuit 31 adds the outputs of shaping circuits 23, 27 and 30 and the reference voltage, and inputs them to a clamp circuit 33. Disconnection of the cable can be detected with the aid of a change in the output voltage VB of the clamp circuit 33.