To provide an estimation type fault method which has its short execution time by comparing the average square error value showing the SN ratios of the 1st and 2nd reading operations with each other and defining the difference between both error value as a change of a data storage system.

The 1st average square error value (MSE value) is calculated for a 1st reading operation (30) and stored in a memory (35). Then the 2nd MSE value is calculated for a 2nd reading operation (40), and the 1st and 2nd MSE value are comparison with each other (45). When both MSE value are coincident with each other (50), it's decided that the head floating height has no change. Thus, a test is over. If a difference is confirmed between both MSE value (50), it's decided that a data storage system has a change (55). Then it's decided whether the system change may possibly cause a fault or not (60).