An antenna device of an embodiment includes: an antenna; a variable impedance matching circuit that is connected to the antenna; a probe that receives power from the antenna; a power detector that is connected to the probe; a control circuit controlling the variable impedance matching circuit based on a value of power measured by the power detector; a first arithmetic circuit calculating a variation in the value of power measured by the power detector; a comparator circuit that compares the variation with a predetermined numerical value range; and a starter circuit that activates the control circuit based on a result of the comparison performed by the comparator circuit.