In mobile communications, it is required that the beam direction is maintained to track the desired direction as the mobile is moving. For such a purpose, the mobile includes an angular rate sensor mounted therein which detects the state of turn in the mobile and to control the beam direction of the antenna in accordance with the state of turn as well as the strength of radiowave received by a receiver in the mobile. Antenna elements are in the form of microstrip antenna and are arranged in plane on the same dielectric substrate. Feeding and drive circuit layers for controlling the transmission and reception at the antenna elements are stacked into a single layered unit. This enables the antenna system to be formed into a low-profile structure. The dielectric substrate of the microstrip antenna element is formed by stacking a plurality of dielectric substrate different in dielectric constant from one another. It is thus intended that the band width of the antenna is increased and that the mutual coupling between the antenna elements is reduced to prevent the gain of the antenna from being lowered. Furthermore, the position of feed points in the antenna element are rotated against each adjacent antenna element. This can improve the axial ratio in the array antenna over a wide band width.