PURPOSE: To restore a data transmitting equipment not returning a response during the period from ended polling up to the succeeding polling without exerting influence upon other data transmitting equipments by transmitting a response demand sequence to the data transmitting equipment concerned.

CONSTITUTION: At the time of starting poling in accordance with a new polling command after completing data transmission from a data transmitting equipment 5, a control station 1 checks whether other data transmitting equipments have been registered or not. If a data transmitting equipment 4 has been registered but a data transmitting equipment 3 next polling has not been registered yet, the control station 1 executes poling to the equipment 3. At the time of receiving a cancel character sequence again from the equipment 3, the control station 1 refers to a response demand table and transmits a response demand sequence to the equipment 4. Consequently, the data transmitting equipment concerned is restored without exerting influence upon other data transmitting equipments.