An antenna device comprising: an insulating substrate main body; a ground plane which is patterned with metal foil on the substrate main body; an antenna-occupied area that is provided in contact with one side of the substrate main body on the substrate main body as an area in which the ground plane is not formed; a slit section that is formed in the ground plane so as to extend from the antenna-occupied area in the direction opposite to the one side of the substrate main body; a power feeding pattern that is patterned with metal foil so as to extend into the slit section, provided with a power feeding point at the base end side, and connected with a first passive element halfway while the tip end side extends into the antenna-occupied area toward the one side of the substrate main body; an antenna element of a dielectric antenna that is constituted by a dielectric base body, a conductor pattern formed on the surface of the dielectric base body, and a pair of electrodes (4a, 4b) which are connected to each other by the conductor pattern and are formed at both ends of the dielectric base body, the antenna element being placed along the one side of the substrate main body while one of the electrodes is connected to the tip end of the power feeding pattern; a second passive element that is connected between the other electrode of the antenna element and the ground plane adjoining thereto; and a ground connection pattern having an inductance that is patterned with metal foil, connects the tip end of the power feeding pattern with the ground plane opposite to the antenna element whereby the power feeding pattern includes a power feeding-side land portion at the tip end to which the one of the electrodes of the antenna element is connected and a fine line portion provided between the power feeding-side land portion and a portion at which the first passive element is connected; the width of the power feeding-side land portion is formed wider than that of the fine line portion and the distance between the power feeding-side land portion and the ground plane adjoining thereto is set narrower than that between the fine line portion and the ground plane. an insulating substrate main body; a ground plane which is patterned with metal foil on the substrate main body; an antenna-occupied area that is provided in contact with one side of the substrate main body on the substrate main body as an area in which the ground plane is not formed; a slit section that is formed in the ground plane so as to extend from the antenna-occupied area in the direction opposite to the one side of the substrate main body; a power feeding pattern that is patterned with metal foil so as to extend into the slit section, provided with a power feeding point at the base end side, and connected with a first passive element halfway while the tip end side extends into the antenna-occupied area toward the one side of the substrate main body; an antenna element of a dielectric antenna that is constituted by a dielectric base body, a conductor pattern formed on the surface of the dielectric base body, and a pair of electrodes (4a, 4b) which are connected to each other by the conductor pattern and are formed at both ends of the dielectric base body, the antenna element being placed along the one side of the substrate main body while one of the electrodes is connected to the tip end of the power feeding pattern; a second passive element that is connected between the other electrode of the antenna element and the ground plane adjoining thereto; and a ground connection pattern having an inductance that is patterned with metal foil, connects the tip end of the power feeding pattern with the ground plane opposite to the antenna element whereby the power feeding pattern includes a power feeding-side land portion at the tip end to which the one of the electrodes of the antenna element is connected and a fine line portion provided between the power feeding-side land portion and a portion at which the first passive element is connected; the width of the power feeding-side land portion is formed wider than that of the fine line portion and the distance between the power feeding-side land portion and the ground plane adjoining thereto is set narrower than that between the fine line portion and the ground plane.