PURPOSE: To prevent an output of an error signal by providing an inaudible sound wave generating circuit to the detector, providing a sound collection device collecting the sound wave to the handset, and detecting the hook state based on a level of a signal outputted from the sound collection device.

CONSTITUTION: With a handset 7 loaded to a telephone set main body 1, since a sounding body 6 and a sound collection device 8 are close to each other, a level of a signal collected by a sound collection device 8 exceeds a reference level set to a hook detection circuit 10. Thus, since a detection signal representing on-hook is outputted from the circuit 10 to a telephone set circuit 3, the circuit 3 receiving the signal keeps the on-hook state. On the other hand, when the handset 7 is lifted up and a sounding body 6 and the sound collection device 8 are parted, the level of the signal collected by the sound collection device 8 is reduced more than the reference level. Thus, since a detection signal representing the off-hook is outputted from the circuit 10 to the circuit 3, the circuit 3 receiving it is connected to the telephone line 2.