PURPOSE: To prevent the erroneous judgment of a failure detecting device by comparing the estimated attitude angle estimated with the attitude angel detected by an optical sensor and the optical sensor attitude angle output signal, and judging the optical sensor as 'defective' or 'normal' based on the compared result.

CONSTITUTION: A comparison section 5 is inputted with the estimated attitude angle estimated by an attitude angle estimation section 4 and the attitude angle output signal from an optical sensor 1 and outputs the difference. The difference is inputted to an attitude angle limit check section 2, if the difference exceeds threshold of the check section 2, the status '1' is outputted to a failure detecting processing section 3. If the difference does not exceed the threshold, the status '0' is outputted to the processing section 3. The processing section 3 is inputted with the status from the check section 2, it judges the sensor 1 as 'defective' when the status is '1', and it judges the sensor 1 as 'normal' when the status is '0'. The erroneous judgment on the optical sensor 1 can be prevented.