PURPOSE: To easily execute the maintenance operation of a coprocessor by referring to a stop main cause table at the time of the stop completion notice of the coprocessor and reserving power source turn-off when a stop main cause is owing to the depression of a stop key, so as to set the coprocessor in a stop state.

CONSTITUTION: When the stop key 12 is depressed, for example, an emulating program 9 requests the coprocessor to stop, and a process program 6 on the side of the coprocessor 1 responds the stop completion notice to the emulating program 9 after a stop processing. Thus, a judgement part 10 refers to the stop main cause table 8 and notices stop completion when the stop main cause is the depression of the stop key 12. Consequently, the power source is not cut-off and the stop state of the coprocessor is set. Thus, the maintenance operation of the coprocessor 1 becomes easier.