To provide a transmission line disconnection detection method that enables the disconnection of transmission lines to be accurately detected in a control and monitoring signal transmission system that uses a transmission synchronization method. A management data area, which differs from a control and monitoring data area constituted by data for control data signals and data for monitoring data signals, is provided to transmission signals transmitted by a common data signal line. A substation captures input information from an input unit corresponding to a local station, captures the control data for referencing an output unit of another station, which corresponds to the aforementioned input unit, from the transmission signals, and obtains the timing of a pseudorandom output change, which is equivalent to the timing of a true output change of the output unit, on the basis of the control data. If a time difference from the timing of the pseudo random output change to the timing of an input change of the input unit is smaller than a first threshold value, a signal composed of data representing a first fault condition is superimposed on the management data area, and if said time difference is larger than a second threshold value, a signal representing a second fault condition is superimposed on the management data area.