PURPOSE: To back up the master station without fail in the case an abnormality occurs inside and outside the network.

CONSTITUTION: A trunk line 281 is connected to a master station MS and a trunk line 282 to a sub master station SMS. At the location of a frame 21 to be sent on a ring type network 20, the first information indicating whether the frame 21 should be sent from a master station MS or from a sub master station SMS and the second information indicating either of the external clock or the internal clock is selected from a trunk line 28 is written. The master station MS and the sub master station SMS are provided with detection means 25, monitoring means 26, and control means 27 to detect whether the frame 21 is normally received and whether the external clock is normal, determining whether serving as a cycle master or being a slave station by monitoring the first and second information.