PURPOSE: To prevent the occurrence of a faulty state by counting the retrial frequencies for reading a disk, then counting out the succeeded frequencies of the retrial beyound a prescribed frequency or more and giving an alarm when the success frequencies exceed a prescribed level.

CONSTITUTION: A microprogram stored in a flexible disk 2 is read out through a control part 1 and loaded to a control memory 3. When the reading of the disk 2 fails, a retrial is carried out. The retrial frequencies are counted 6, and then the success frequencies of the retrial with a prescribed frequency (n) or more are counted 7. When the success frequencies exceeds a prescribed level (m), the alarm information C is delivered outside a data processor. In such way, the defect of the disk can be known previously, thus occurrence of a fault in the data processor can be prevented.