PURPOSE: To discriminate whether a correctable error is a tentative error or a fixed error, by transmitting an error report signal when the number of error correction times reaches a specified value and producing the error report signal again if there comes further error correction within a prescribed time after reaching the value.

CONSTITUTION: The correctable error of a micro instruction data 9 read out from a cotrol storage device 3 is detected at an error detection control circuit 4, a signal 10 after error correction is transmitted to an operation control section 6 and the number of times of error correction is counted 7. When the number of times of error correction reaches a specified value, the counter 7 controls an error report control circuit 8 to transmit an error report signal 17 to a maintenance diagnostic device 2 and to reset the counter 7. If a new correctable error is generated within a prescribed time monitored at a time monitor circuit 30, it is regarded as a fixed failure and the circuit 8 suppresses the generation of the further error report signal. If there comes no error even once within the monitored time, it is taken as a tentative error and the device 2 collects error log information 18 from a control section 6.