An interface circuit provided correspondingly to a port of a main device including a telephone exchange function, the port allowing connection of a telephone or an intercom, the interface circuit comprising: a power supply circuit adapted to supply necessary power supply voltage according to the telephone or the intercom connected to the port; and a state detection circuit adapted to detect, 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 a value of an input impedance of the telephone, an adapted to detect, 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 an input impedance of the intercom, wherein the power supply circuit includes a voltage circuit adapted to generate the necessary power supply voltage according to the telephone or the intercom, a first constant current circuit that is connected between one line of two lines of the port and the voltage circuit, and a second constant current circuit that is connected between another line of the two lines of the port and a ground potential, wherein the voltage circuit includes a first transistor that is inserted between a first voltage supply that outputs negative voltage according to the telephone and the first constant current circuit, and that has a control terminal to which output voltage of the first voltage supply is supplied, a second transistor, a first zener diode and a second zener diode that are inserted in series between a second voltage supply that outputs higher positive voltage than the first voltage supply and the control terminal of the first transistor, and a third transistor that is inserted between a connection node of the first zener diode and the second zener diode and the second voltage supply, wherein the second transistor and the third transistor are each turned off when the telephone is connected to the port, wherein the second transistor and the third transistor are each turned on when the intercom is connected to the port and the intercom is in the standby state, and wherein the second transistor is turned on and the third transistor is turned off when the intercom is connected to the port and is in the activation button pressed state. a power supply circuit adapted to supply necessary power supply voltage according to the telephone or the intercom connected to the port; and a state detection circuit adapted to detect, 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 a value of an input impedance of the telephone, an adapted to detect, 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 an input impedance of the intercom, wherein the power supply circuit includes a voltage circuit adapted to generate the necessary power supply voltage according to the telephone or the intercom, a first constant current circuit that is connected between one line of two lines of the port and the voltage circuit, and a second constant current circuit that is connected between another line of the two lines of the port and a ground potential, wherein the voltage circuit includes a first transistor that is inserted between a first voltage supply that outputs negative voltage according to the telephone and the first constant current circuit, and that has a control terminal to which output voltage of the first voltage supply is supplied, a second transistor, a first zener diode and a second zener diode that are inserted in series between a second voltage supply that outputs higher positive voltage than the first voltage supply and the control terminal of the first transistor, and a third transistor that is inserted between a connection node of the first zener diode and the second zener diode and the second voltage supply, wherein the second transistor and the third transistor are each turned off when the telephone is connected to the port, wherein the second transistor and the third transistor are each turned on when the intercom is connected to the port and the intercom is in the standby state, and wherein the second transistor is turned on and the third transistor is turned off when the intercom is connected to the port and is in the activation button pressed state.