PURPOSE: To quickly decide abnormality while preventing misjudgement by more shortening a time required for deciding a sensor output as abnormal, as difference between the sensor output and a threshold is larger.

CONSTITUTION: When the absolute value of the difference between the sensor output VPV of a steering torque sensor and an upper limit and a lower limit thresholds VH, VL are obtained as departure degree (d) and an abnormality judgement time (t) corresponding thereto is retrieved, if the time (t) is determined as t4 at a T2 points, a timer starts, and if the time (t) is determined as t3 at a T3 point, the timer restarts. After the T3 point, if the departure degree (d) settles between d2 and d3, the time (t) becomes up at a TE3 point after the lapse of t3 time so that the output VPV is decided as abnormal confirming the same so as to stop the control of assist force. In this case, when the output VPV is close to the threshold VL (VH), the time (t) is set slightly longer, and is set shorter as parting from the threshold VL (VH). Therefore, when the steering torque sensor is lost, the abnormality of the output VPV can be decided early so that the control of the assist force can be stopped early.