PURPOSE: To shorten processing time for avoiding deadlock and to prevent the use efficiency of a processor from deteriorating.

CONSTITUTION: The number of messages accumulated in the message buffer 13 of a calculation processor 2 is checked. When the messages cannot be accumulated in the message buffer 13, the messages which are transmitted hereafter are returned to a transmission processor 1. Thus, possibility that deadlock in the calculation processor 2 occurs is eliminated. When the returned message is received, the new generation of the message in the transmission processor 1 is temporarily stopped, and the message accumulated in the message buffer 12 is retransmitted after a situation becomes that where deadlock does not occur. When the messages accumulated in the message buffer 12 are eliminated, the generation of the message is resumed.