PURPOSE: To detect output abnormality in a hardware manner, by comparing the logic levels of the output data of the input point and output terminal of an output buffer.

CONSTITUTION: When output is normal, the logic level of the output data S1a read from an output port register 11 becomes same to that of the logic level of the signal S2a appearing in an output terminal 13a. Therefore, the output level of an exclusive OR gate 14a comes to 0 and the signal level appearing in the output terminal Q of FF16a also comes to 0 and, therefore, the content of an output abnormal register 17 is 0 at it is. When output is abnormal, since the level of the signal S2a is 0 when 1 is read as the output data S1a, the signals at both terminals of a buffer 12a comes to levels different from each other. As a result, the output level of the OR gate 14a comes to 1 and the signal level appearing at the terminal Q of FF16a also comes to 1. Then, 1 is set to the memory region concerned of a register 17 and an interruption processing signal INT is outputted through an OR gate 15 and output abnormality is detected by relatively simple constitution.