PURPOSE: To avoid the dead lock of a system by preliminarily setting the number of messages which can exist in a message queue.

CONSTITUTION: When a message output request is issued, a message queue discriminating means 2 discriminates the number of messages in the message queue. If this number is smaller than a preliminarily set prescribed number, a message queue register means 3 is started; but if this number is equal to the prescribed number, a message queue retrieving means 5 is started, and a message dequeue 6 is started in the case of the existence of the same message in the message queue, and the message queue register means 3 is started at the time of discriminating the end of retrieval. Thus, the state that the same message is repeatedly outputted because of the frequent occurrece of trouble to make message decoding difficult or the state that the message queue overflows because of the frequent occurrence of messages in a short time to delete required messages is prevented to prevent the system from falling into the dead lock state.