A torque detection portion includes a first arm having a detection coil L1 and a resistor R1, and a second arm having a detection coil L2 and a resistor R2. When a test mode is set, a reference voltage V.sub.ref having an abnormal value is outputted to a monitoring portion 13. An abnormal condition is intentionally caused in the monitoring portion 13. A main torque signal V.sub.tm is compared with a sub torque signal V.sub.ts in the monitoring portion 13. When the reference voltage V.sub.ref set in the monitoring portion 13 has the abnormal value, a sub torque signal V.sub.ts having a level of 0 (V.sub.ts=0) is forcibly outputted in a normal condition. Then the signal V.sub.tm is equal to the signal V.sub.ts, the monitoring portion 13 determines that a shortcircuit occurs between a main torque signal line and a sub torque signal line.