An antenna device has a core made of a magnetic material, a coil formed by winding a conducting wire on a predetermined range including the core and the outer region of the core, and a sub-core that adjusts the resonance frequency by moving in the inner peripheral region of the coil and in the direction orthogonal to the winding direction of the coil. A communication system of the present invention is a combination of an external communication device, the antenna device, and an internal communication device that is connected to the antenna device and is disposed in a cabin.