PURPOSE: To preferentially execute the communication of a higher degree of urgency by holding the priorities of data and a transmission request respectively in a priority register for communication and the priority register for a request during the communication, and allowing an interruption processing circuit to acquire a communication channel by comparing them by a priority comparison circuit.

CONSTITUTION: The priority of a communication request from a higher order hierarchy is set previously in the priority register 42 for a request through a transmission channel access device 22. If the communication channel is not idle, the priority of the communication being in progress at present held in the priority register 42 for communication and the priority of the communication request from the higher order hierarchy held in the priority register 42 for a request are compared by the priority comparison circuit 43, and if the priority of the request is higher, the interruption processing circuit 44 sends an interruption signal to a signal line 11. Thus, the communication of the higher degree of urgency is preferentially executed.