Abstract: Disclosed are a built-in dual band antenna device and an operating method thereof in a mobile terminal. In the built-in antenna dual band antenna device, a built-in dual band antenna has a first conductive antenna pattern formed on a board extended from the upper side of a main PCB and a second conductive antenna pattern on a board extended at a right angle from the upper side of the main PCB. A whip antenna is connected to the built-in dual band antenna, and contained in the mobile terminal when the whip antenna is retracted. A whip antenna driver extends or retracts the whip antenna. A duplexer separates an RF signal received from the built-in dual band antenna from an RF signal to be transmitted to the built-in dual band antenna. A controller processes the RF signals received at and transmitted from the duplexer and controls the whip antenna driver to extend the whip antenna in a speech state or upon a call attempt from a user.