PURPOSE: To avoid deterioration in speech quality by detecting an on-hook state or an off-hook state at an extension terminal depending on the result of comparison between a specific reference voltage and a terminal voltage of the extension terminal.

CONSTITUTION: A constant voltage diode 23 of a hook detection section 26 generates a reference voltage Vref stabilized into a Zener voltage by impressing a DC power supply voltage V,, to its cathode terminal via a resistor 24. The reference voltage is set to be a nearly intermediate voltage level between the terminal voltage of a slave set in the case of hook-on and a terminal voltage when the slave set 12 is hooked off. Furthermore, the reference voltage Vref is impressed to an inverting input terminal of a comparator 25 and a terminal voltage Vp of the slave set 12 provided with a circuit impedance ZR2 is impressed to a noninverting input terminal to detect a state of on-hook or off-hook at the slave set 12 depending on the result of comparison. Thus, mismatching of the circuit impedance between a master set and the slave set is eliminated to avoid deterioration in speech quality.