An antenna device includes a rectangular conductor pattern that is disposed substantially in parallel to a ground plate at a predetermined distance, a short-circuit portion that electrically connects the conductor pattern to the ground plate, a first feeding point for transmitting and receiving a signal of a first frequency, and a second feeding point for transmitting and receiving a signal of a second frequency. An electric length of one side of the conductor pattern is set to half a wavelength of the second frequency. The short-circuit portion is disposed in the center portion of the conductor pattern, and an area of the conductor pattern forms a capacitance that resonates in parallel with an inductance provided in the short-circuit portion at the first frequency.