PURPOSE: To easily know the generation of a fault by supervising the interval of a data frame with a secondary station, and when the interval exceeds the prescribed interval, detecting an excess with the secondary station.

CONSTITUTION: A timer means 24 having the timer time approximately the same as the timer time of a timer means 13 on a primary station 1 side and a control means 25 supervising the interval between the data frames received with the use of the timer means 24 are provided on a secondary station 2 side. Since the data frames are periodically and successively transmitted at the same intervals as the timer time of the timer means 13, normally, the interval between the received data frames is approximately the same as the timer time of the timer means 13. However, when the fault such as the abnormality of the primary station 1 or the disconnection of the line is generated, and the interval between the data frames received by the secondary station 2 exceeds the timer time of the timer means 24, it is detected by the control means 25. Thus, the fault generation can be known.