There is provided a hot-plug signal detecting apparatus capable of surely detecting change of a hot plug signal without increasing load on a transmission processing unit. A hot plug signal from a sink device is inputted to an AD input terminal of a controller, the hot plug signal is subjected to AD conversion, and a high level or low level is determined from the digital value, and thus, a hot-plug detecting circuit is not necessary, which can simplify a circuit configuration. Moreover, if a period of the low level of the hot plug signal is shorter than Tout, a period of the low level is prolonged to Tout, which allows the transmission processing unit to surely detect the low level without shortening a cycle at which the hot plug signal is detected.