A method for detecting a failure in a control/monitoring signal transmission system in which a master station is connected with a plurality of slave stations (4, 6, 7) via a common data signal line (DP, DN), transmission of data is performed according to a transmission synchronization scheme, and a transmission signal is transmitted via the common data signal line (DP, DN) and includes a management data region different from a control/monitoring data region comprising data of a control data signal and data of a monitoring data signal the method comprising: acquiring, by a first slave station (4, 7) among the plurality of slave stations (4, 6, 7), input information from an input part corresponding to the first slave station (4, 7) as well as acquiring control data from the transmission signal, the control data being referred by the first slave station (4, 7) and being for an output part of the first slave station or a second slave station among the plurality of slave stations (4, 6, 7), the output part being in a correspondence relation with the input part; and characterized by obtaining, by the first slave station (4, 7), a pseudo output change timing equal to a true output change timing of the output part based on the control data; comparing, by the first slave station (4, 7), a time difference between the pseudo output change timing and an input change timing of the input part with a first threshold value and a second threshold value; determining, by the first slave station (4, 7), a normal state when the time difference is larger than the first threshold value and smaller than the second threshold value, a first failure state when the time difference is smaller than the first threshold value, and a second failure state when the time difference is larger than the second threshold value; and causing, by the first slave station (4, 7), a signal configuring data indicating the first failure state or the second failure state to be superimposed on the management data region. acquiring, by a first slave station (4, 7) among the plurality of slave stations (4, 6, 7), input information from an input part corresponding to the first slave station (4, 7) as well as acquiring control data from the transmission signal, the control data being referred by the first slave station (4, 7) and being for an output part of the first slave station or a second slave station among the plurality of slave stations (4, 6, 7), the output part being in a correspondence relation with the input part; and characterized by obtaining, by the first slave station (4, 7), a pseudo output change timing equal to a true output change timing of the output part based on the control data; comparing, by the first slave station (4, 7), a time difference between the pseudo output change timing and an input change timing of the input part with a first threshold value and a second threshold value; determining, by the first slave station (4, 7), a normal state when the time difference is larger than the first threshold value and smaller than the second threshold value, a first failure state when the time difference is smaller than the first threshold value, and a second failure state when the time difference is larger than the second threshold value; and causing, by the first slave station (4, 7), a signal configuring data indicating the first failure state or the second failure state to be superimposed on the management data region.