PURPOSE: To increase the number of items of information which can be transmitted, by transmitting relative measureing information on priority basis together with status information being changed in state, when the contents of the status information change.

CONSTITUTION: A synchronizing signal, and information words W1∼WN are transmitted cyclically. If the change in state of W10 as status information (SV) is detected while W1 is transmitted, W10 is preferentially transmitted after W1 and as to Wm(TMm) and Wn(TMn) which are measurement data (TM) relating to W10, priority transmission by twice each is assigned by a priority transmission item assigning circuit and priority transmission frequency assigning circuit (which are not both shown in the figure), indicating the completion of the priority transmission.