PURPOSE: To realize a loop transmission system which has an improve circuit service and a simplified procedure of the transmission protocol, by applying a transmission procedure eliminating a retry due to the nonreply.

CONSTITUTION: A data transmission/reception control part 307 gives a start to a circuit control part 309 when a command is given to the transmission of data. Thus the control part 309 controls a synchronizing frame circuit 301, an S-P converter 302 and a P-S converter 303 in place of a data transmission control part 307 during the data transmission service of the part 307. Therefore the information transmitted from another station and showing a data transmission request is stored in a receiving submemory 310. The control part 309 receives the information showing the data transmission request from the submemory 310 and then sends the answer information showing that a receiving memory 304 of the own station is busy back to the station that gave the above-mentioned transmission request.