An antenna system includes a case, an antenna configured for a first communication system, an inside orthogonal antenna configured for a polarization diversity communication system, and an outside orthogonal antenna configured for the polarization diversity communication system. The antenna for the first communication system and the inside orthogonal antenna are disposed inside the case such that a main radiation polarization plane of the inside orthogonal antenna is orthogonal to a main radiation polarization plane of the antenna for the first communication system. The outside orthogonal antenna is disposed outside the case such that a main radiation polarization plane of the outside orthogonal antenna is orthogonal to the main radiation polarization plane of the inside orthogonal antenna.