PURPOSE: To adopt or reject information of other party based on presence or absence of the right of transmission by allowing a slave station given with the right of transmission from a master station to designate other slave station and using two lines between slave stations for transmission and reception.

CONSTITUTION: When the master station M receives a data burst overhead signal DBOH1 from the slave station C1, the station M transmit a frame reference burst FRBOH2 representing whether conversation is made possible or not to the slave station C2 in the next polling timing. When the slave station C2 responds to it and transmits the data burst DBOH2 representing that the conversation is enabled to the other slave station. The master station M outputs a signal FRBOH3 representing the permission that mutual communication is enabled between slave stations C1 and C2 in the next polling period so as to intermit once the polling. Thus, the slave stations C1 and C2 use the two lines so as to attain transmission/reception mutually.