PURPOSE: To attain the self-diagnosis for the working of an ECC (error detection/correction) circuit with no use of a fixed disk device, by providing a buffer circuit to store data by a data switching circuit in a diagnosis mode and destructing the data partly by a control circuit.

CONSTITUTION: When the working of an ECC circuit 5 is diagnosed, the data flowing route is first changed by a data switching circuit 7 so that the data stored in a data buffer circuit 2 is delivered to a buffer circuit 6. Then the data stored in the circuit 6 is destructed partly by a control circuit 1, and the data destructed partly is supplied to the circuit 2 from the circuit 6 and then compared with the first data. If coincidence is obtained from said comparison, it is decided that the circuit 5 is adding a check bit to the output data and checking and correcting an error of the input data in a normal way.