The present invention achieves size reduction while maintaining good antenna performance by reducing mutual interference between antennas when equipped with a plurality of antennas in a common case 5.When an antenna device is equipped with an AM/FM antenna 30 as a first antenna and an SDARS antenna 40 or a GPS antenna 50 as a second antenna which are provided in a common case and have frequency bands different from each other, an additional conductor part 37 is extended from a conductor main body part 36 of a capacitive element 35, and the additional conductor part 37 has a parallel belt-shaped part 37a extending parallel to the conductor main body part 36 and having a length equal to a quarter of an effective wavelength in the frequency band of the second antenna.