A transponder for performing bidirectional conversion between a client-side signal used for communication to a client device and a transmission path-side signal used for communication to a transmission path, the transponder comprising: a client-side interface for inputting/outputting the client-side signal; a transmission path-side interface for inputting/outputting the transmission path-side signal; a connection determining unit for determining which of the another client-side interface and the client device is coupled to the client-side interface; and a transmission signal generating unit for outputting, in a case where a state signal indicating a state on a transmission path side is input to the transmission path-side interface, a transmission signal indicating that the state signal has been input, from the client-side interface, wherein: the transmission signal generating unit changes a form of the transmission signal to be output from the client-side interface, based on a determination result obtained by the connection determining unit.