An antenna device to be installed on a predetermined portion of a vehicle body, comprising: an inner case (13, 23) configured to form a housing space, and an outer case configured to cover the inner case (13, 23); an electronic component provided in the housing space; and a first antenna element (14, 24a) provided between an outer surface of the inner case (13, 23) and an inner surface of the outer case and having a through hole, wherein the first antenna element (14, 24a) is electrically connected to the electronic component while maintaining water-tightness of the housing space inside the inner case (13, 23), and further comprising a watertight connecting member (16, 26a) embedded in the inner case (13, 23), wherein the watertight connecting member (16, 26a) has an outer terminal exposed on the outer surface of the inner case (13, 23) which serves as a mounting mechanism for the first antenna element (14, 24a) and an inner terminal exposed on the inner surface of the inner case (13, 23) which is conductive with the outer terminal while maintaining the water-tightness of the housing space inside the inner case (13, 23), wherein one dent portion is provided on a part of the top portion of the inner case (13, 23), and another dent portion which corresponds to the one dent portion is provided on a top portion of the first antenna element (14, 24a), and wherein the outer terminal is in contact with the one dent portion of the inner case (13, 23) and with the another dent portion of the first antenna element (14, 24a), wherein the antenna device includes a second antenna element having a joint plate, wherein a male screw made of a metal is threadably fitted through a through hole of the joint plate of the second antenna element so that the joint plate is sandwiched between a top portion of the male screw and the inner terminal, and a male screw made of a metal is threadably fitted through the through hole of the first antenna element (14, 24a) so that the first antenna element (14, 24a) is sandwiched between a head portion of the male screw and the outer terminal. an inner case (13, 23) configured to form a housing space, and an outer case configured to cover the inner case (13, 23); an electronic component provided in the housing space; and a first antenna element (14, 24a) provided between an outer surface of the inner case (13, 23) and an inner surface of the outer case and having a through hole, wherein the first antenna element (14, 24a) is electrically connected to the electronic component while maintaining water-tightness of the housing space inside the inner case (13, 23), and further comprising a watertight connecting member (16, 26a) embedded in the inner case (13, 23), wherein the watertight connecting member (16, 26a) has an outer terminal exposed on the outer surface of the inner case (13, 23) which serves as a mounting mechanism for the first antenna element (14, 24a) and an inner terminal exposed on the inner surface of the inner case (13, 23) which is conductive with the outer terminal while maintaining the water-tightness of the housing space inside the inner case (13, 23), wherein one dent portion is provided on a part of the top portion of the inner case (13, 23), and another dent portion which corresponds to the one dent portion is provided on a top portion of the first antenna element (14, 24a), and wherein the outer terminal is in contact with the one dent portion of the inner case (13, 23) and with the another dent portion of the first antenna element (14, 24a), wherein the antenna device includes a second antenna element having a joint plate, wherein a male screw made of a metal is threadably fitted through a through hole of the joint plate of the second antenna element so that the joint plate is sandwiched between a top portion of the male screw and the inner terminal, and a male screw made of a metal is threadably fitted through the through hole of the first antenna element (14, 24a) so that the first antenna element (14, 24a) is sandwiched between a head portion of the male screw and the outer terminal.