To electrically vary the directivity of a microstrip antenna, which has a base plate formed on the backside of a dielectric substrate and many radiation elements formed on the top surface side, without reducing antenna gain.

Signal transmission lines L1 to L4 which are coupled with the respective radiation elements S1 to S4 in terms of AC and disconnected in terms of DC are provided and bias voltage control circuits M1 to M4 apply a bias voltage selectively to the signal transmission lines L1 to L4 through bias lines K1 to K4. Consequently, the specific dielectric constant of the dielectric substrate 23 in the area between the signal lines L1 to L4 and base plate 22 varies to cause propagated high-frequency signals of the signal transmission lines L1 to L4 to shift in phase, thereby actualizing a radiation pattern by which desired directivity can be obtained.