PURPOSE: To improve the communication efficiency and to monitor the communication content by deciding a single communication line between optional controllers and designating a sender controller and other controller than the sender controller as a transit destination so as to decide a communication path of sender transit destination.

CONSTITUTION: When a packet is sent from a controller (Cr) 3 to a Cr 4 and when a transit ID in its own possession transit destination reference table (TB) is -1, the Cr3 selects a path of Cr3 → 2 → 4 as the transit destination is not designated. When a Cr1 requires communication monitor between the Cr3, 4, the Cr1 gives a command to write a sender destination ID of the TB of the Cr3 to be 4 and a transit ID to be 1. The Cr3 writes the transit ID of a header to be 1 based on the TB and the packet, transfers the packet to the Cr1 from the Cr2 and the Cr1 processes data. Then the Cr1 writes -1 to a transit destination ID of the header, invalidates its own transit destination to set a path of Cr2 → 4. Thus, the communication efficiency is improved and the communication content is monitored.