PURPOSE: To avoid such a case where the serious fault data is erased by the subsequent superscription by inhibiting selectively the superscription to a specific recording area where the comparatively important fault data are written in accordance with the importance of fault data.

CONSTITUTION: When the A fault is decided by reference to the information on the fault occurred sent from a CPU, a 1st table part 301 is read out of a control table and either one of 2nd table parts 31W34 that is shown by an address stored in a field 301C is read out. Then the fields 31aW34a formed at the head parts of those tables 31W34 are tested. When '1' is obtained from said test, the A-rank importance is decided and a signal is sent to the CPU to show the end of the recording process of the fault data. At the same time, no fault data is written into a disk device. While the position information 31cW34c and the length information 31dW34d on the disk device are sent to a disk control part in case '0' is obtained from said test. Then the fault data is written into the disk device and at the same time '1' is stored in the fields 31aW34a.