A first control circuit section (master station) and a second control circuit section (substation) communicate with each other via series-parallel converters. The master station includes regular transmission device and transmission-permitting signal generation device; and the sub station includes regular report device, confirmation reply device relative to a transmission data from the master station, and an unprocessed data table. The unprocessed data table prevents jam-up in upstream communication from the sub station to the master station, enabling to carry out timely regular transmission and regular reporting. Transmission from the sub station to the master station is performed based on a transmission-permitting control signal that the master station generates. Consequently, it is possible to execute communication error processing and diminish communication control burden on the master station while properly performing the regular transmission and regular report between the master station and sub station.