PURPOSE: To adjust a half power angle uniquely decided to one array antenna by providing a wall-shaped projection for reflecting radio waves at a position symmetrical to right and left sides with the center line of the array antenna as an axis.

CONSTITUTION: A wall 4 is provided with a material for reflecting radio waves at the position symmetrical to right and left sides with the center line of the array antenna as the axis, and waves 6 reflected on the wall 6 are synthesized with waves 5 to be directly radiated. When the phase of the reflected wave 6 at that time is not same as that of the radiated wave 5, electric field strength is weakened, and the distribution of the electric field is made unequal at the end of the antenna and at the center. In this case, by moving the wall-shaped projection 4 for reflecting radio waves to right and left, the phase of the reflected wave 6 is changed and as the result, the distribution of the electric field strength due to the synthesization with the radiated wave 5 is changed. By providing the wall-shaped projections 4 for reflecting radio waves at the positions symmetrical in right and left sides, the beam is made symmetrical since the same operation occurs at the positions symmetrical in the right and left sides. Thus, the beam is made symmetric and can be formed at a desired half power angle.