PURPOSE: To quicky execute the display/printing or the like of information with high urgency by successively transmitting data in a transmission buffer with higher priority by a monitoring terminal equipment when a monitoring center device requests data.

CONSTITUTION: Transmission work initializing processing such as priority level L=1 is executed. Then a painter in a buffer management table 10 corresponding to a level L is checked and the existence of data held at the processing level L is judged. When the data exist, data are extracted from a transmission buffer and the painter is updated. The extracted data are edited as a transmission text and returned to a master station and the normal arrival of data is checked by receiving ACK. When there is no transmission data, the priority level L is updated and data in a transmission buffer with the succeeding priority level are transmitted. After completing the data transmission of all transmission buffers, information indicating no text is finally transmitted to end the transmission. Consequently data can be successively processed in the ascending order of priority.