PURPOSE: To make the circumstances of a communication state exact by giving issue sequence information for showing an issue sequence to a communication telegraphic message transmitted and received between plural CPUs, and storing the communication telegraphic message and the issue sequence information as communication history.

CONSTITUTION: An issue number 1 is given to a communication telegraphic message (a) transmitted to a CPU 2 from a CPU 1 and a communication telegraphic message (d) replied to the CPU 1 from the CPU 2, respectively, and an issue number 1-1 is given to a communication telegraphic message (b) transmitted to a CPU 3 from the CPU 2 and a communication telegraphic message (c) replied to the CPU 2 from the CPU 3, respectively. In such a way, by giving the issue number for showing the issue sequence of the telegraphic message to the communication telegraphic message transmitted and received between the CPUs 1 - 3, and storing its telegraphic message and number as communication history, it is possible to make the circumstances of a communication state exact.