A built-in antenna device for improving the radiation efficiency in a portable wireless terminal having an external metallic part is provided. The built-in antenna device includes a main board, an antenna radiator, and a conductor. The main board has a feed pad for supplying an electrical signal and a ground pad connected to ground. The antenna radiator is connected electrically to the feed pad and the ground pad of the main board to radiate a signal. The conductor connects the metallic part and the ground pad of the main board. The metallic part and the ground pad of the main board are at the same ground potential.