A multi-band built-in antenna for a mobile communication terminal having a main board and a casing for protecting the main board, is disclosed. A transmission line is formed to be spaced apart from one outside surface of the main board by a predetermined interval and configured to include an external conductor, a dielectric, and a central conductor so as to transmit signals. A ground clip is configured to ground the transmission line by fastening the transmission line. A radiator is formed by bending the dielectric and central conductor of the transmission line, other than the external conductor of the transmission line, and is configured to operate in multiple bands. An open stub is connected to the ground clip, is bent a plurality of times, and is configured to be operated in a low frequency band, which is lower than the high frequency band.