A sector antenna apparatus mounted on a vehicle has a casing, in which six horn antennas having apertures over an angular range of 180 degrees and extending radially are accommodated. The proximal ends of the horn antennas are connected to an antenna changeover switch. A portion of the horn antennas which emits beam radiation in the forward and backward direction and diagonal direction of the vehicle have large apertures, and a portion of the horn antennas which emits beam radiation to the right and left of the vehicle have a small aperture. Thus, the required antenna characteristics, such as angular resolution, beam width, antenna gain, are achievable in the required direction.