PURPOSE: To decrease the performance reduction due to the line supervisory function by providing a counter means subjected to initial set at every communication with a slave station and starting the count on a master station.

CONSTITUTION: A preset time (setting time) is set to a comparison means 14, and the count time and the setting time of the 1st count means 15 and the 2nd count means 16 are compared by a comparison means 14. When the count time reaches the setting time, it is informed to a test communication means 13. When the means 13 receives the said notice, test communication is conducted to corresponding slave station 2 respectively. In conducting the test communication in this way, then the test communication is not executed when the substantial communication is frequent, and the line is not occupied. Thus, the performance decrease due to the line supervisory function is reduced.