A dual-band antenna device allowed to perform communication at a first frequency in a predetermined frequency band and at a second frequency in a frequency band higher than the predetermined frequency band includes: a ground conductor; a folded antenna conductor including a first linear part and a second linear part that are caused to face each other at a distance by folding; an LC resonant circuit that is included in the folded antenna conductor, that lets the first frequency pass, and that lets the second frequency attenuate; and a feeding point between the ground conductor and the folded antenna conductor. A narrow gap part is provided between the first linear part and the second linear part of the folded antenna conductor, the narrow gap part measuring a distance shorter than a distance measured in a different portion between the first linear part and the second linear part.