An abnormality determination means performs detection of abnormality of one of the pairs of detection means at a normal speed, and performs detection of abnormality of the other of the pairs at a speed not higher than the normal speed, and, when a sign of abnormality of the detection means is detected at the normal speed, a CPU performs switching to the other normal pair and continues control, and the abnormality determination means performs detection of abnormality of the other normal pair at the normal speed, and meanwhile, continues to perform detection of abnormality of the abnormal pair at a speed not higher than the normal speed.