There is provided an interface circuit according to which a telephone or an intercom may be connected to a port of a main device including a telephone exchange function. The interface circuit includes a power supply circuit that supplies necessary power supply voltage according to the telephone or the intercom connected to the port, and a state detection circuit that detects, in a case where the telephone is connected to the port, whether the telephone is in an on-hook state or in an off-hook state, based on the value of an input impedance of the telephone, and that detects, in a case where the intercom is connected to the port, whether the intercom is in a standby state or in an activation button pressed state, based on a value of the input impedance of the intercom.