A foreign substance detecting circuit may include a voltage detector that may detect a voltage level from a first pin of a connector, a slope detector that may detect a voltage slope indicating a change of the voltage level based two voltage levels from the first pin, and a foreign substance determining device that may determine a presence of a foreign substance in the connector. The foreign substance determining device may determine whether a resistance range of an external device is detected from the at least one first pin based on the detected voltage level. The foreign substance determining device may, when the resistance range is detected, determine that the foreign substance is present in the connector, based on the voltage slope having a value determined to correspond to charge and discharge characteristics due to a capacitance element of the foreign substance.