An antenna device of a mobile terminal for securing a performance of an antenna of the mobile terminal having a case of a metal material is provided. The antenna device of the mobile terminal includes an antenna module for radiating electric waves, and a case for forming an external form of the mobile terminal, made of a metal material, having a slot in a portion of the metal material, and electrically connected to each of the antenna module and a ground of the mobile terminal, and for operating as a radiator through the slot.