The present invention relates to an antenna device and enables the directivity of an antenna element to be changed without affecting the resonance frequency of the antenna element. The antenna device comprises a first grounded conductor, an antenna element mounted on the first grounded conductor via an insulator, a second grounded conductor disposed separate from the first grounded conductor, and a changing unit for changing directivity of the antenna element by adding the second grounded conductor to the first grounded conductor or canceling the addition thereof.