A built-in antenna apparatus of a mobile terminal includes a main board having at least one feeding portion for feeding RF power and at least one grounding portion at ground potential. The antenna apparatus includes first and second thin metal plates configured to be stacked on the main board and spaced from one another. The second metal plate is electrically connected to the feeding portion and has a length sufficient to resonate within a communication frequency band of the mobile terminal The first metal plate is electrically connected to the grounding portion and electromagnetically coupled with the second metal plate to resonate.