In an antenna device, a first helical part of a first antenna and a second helical part of a second antenna is disposed in a dielectric body on a ground plane. Each helical part is helically wound up in a direction perpendicular to the ground plane and includes a plurality of one-turn portions. Each one-turn portion of the first helical part has a peripheral length of M times a wavelength λ of use, where M is a positive natural number. One of the one-turn portions of the second helical part closest to the ground plane has a peripheral length Ks that is N times the wavelength λ of use, where N is a positive natural number greater than M. One of the one-turn portions of the second helical part farthest away from the ground plane has a peripheral length Ke, and (M·λ)