Abstract: There is provided a transmission line switch-over control system capable of significantly improving the nonoperational situation such as circuit disconnection during call or call quality deterioration without conducting unnecessary switch-over of a circuit or a transmission line.A line quality input data handler 5 acquires at least one transmission line quality data out of a received level, a received C/N, and a bit error rate of a normal transmission line 3 and sends it to a forecaster 6. The forecaster 6 produces estimated data of situation of transmission line based on the input quality data of the transmission line. A switch-over controller 7 presumes possibility that the line quality will deteriorate within a predetermined time such as, for example, the mean call holding time. If the presumed possibility of deterioration is larger than a predetermined decision threshold, the switch-over controller effects control so as to switch over the normal transmission line 3 to a back-up transmission line 4.