An antenna device including: an antenna unit having an oscillating body capable of oscillating at a predetermined natural frequency and being displaceable by an external magnetic field, and a converter for converting motion of the oscillating body to an electrical signal, when a radio wave signal having a frequency band for inducing resonance of the oscillating body comes, the oscillating body resonating with a magnetic field component of the radio wave signal, the converter converting resonance of the oscillating body to the electrical signal, and an electrical signal corresponding to the radio wave signal being outputted; a sensitivity varying section capable of varying degree of displacement of the oscillating body occurring by the external magnetic field; and a sensitivity controller for adjusting the degree of the displacement by using the sensitivity varying section in accordance with the electrical signal outputted from the antenna unit.