According to one embodiment, a private branch exchange includes a receiver which receives caller ID information divided into a plurality of packets, the caller ID information specifying a caller side telephone set, when receiving an incoming call, a measurement unit which measures an elapsed time after the receiver has received the packets, a reset unit which resets the elapsed time measured by the measurement unit when the receiver receives the packets, and a transmitter which transmits the packets received by the receiver to a private telephone set on an incoming call side, as caller ID information, when the elapsed time measured by the measurement unit exceeds a specified time.