In an antenna device of the invention used for a wireless LAN, a first substrate with a conductive pattern formed thereon to function as a radiator and a second substrate with conductive patterns formed thereon to function as a wave director and a reflector are held respectively in an inner casing and in an outer casing of the antenna device in a slidable manner. The antenna device is attached to a top face of an access point by means of a three-dimensional joint. The user loosens a screw attached to the outer casing and slides the inner casing relative to the outer casing, so as to change the positional relation of the inner casing to the outer casing. The simple change of the positional relation readily switches over the directional characteristic of the antenna device between a first position where the wave director, the radiator, and the reflector are arranged in parallel across preset distances and the antenna device functions as a Yagi-Uda antenna having high directional characteristic with regard to 2.4 GHz radio frequency signals and a second position where neither the wave director nor the reflector practically works and the antenna device functions as a non-directional antenna.