PURPOSE: To automatically evaluate data at the time of mode switching by providing an expected value part and an actual value part of output data and reading out data based on the result of comparison between them and evaluating output data.

CONSTITUTION: Output data is stored in an output data storage part 22, and the input/output state of data is stored in a flag part 16. In the recording mode, data where a special code is added to the beginning of input/output data is stored in an input/output data control means 14 or 12. In the evaluation mode, output data transferred from a disk device 3 to an output data expected value part 19 and data transferred from the storage part 22 to an output data actual value part 20 are compared with each other by an output data comparing means 21. When the number of data in the expected value part is small, a disk control means 15 reads out data from the disk device 3. Thus, output data is evaluated based on read-out data at the time of switching of a mode setting part 17.