An antenna device includes an antenna unit, a power supply terminal and a time constant circuit. The antenna unit includes an antenna element and a variable capacitance element that is variable in capacitance in accordance with a voltage applied to the variable capacitance element, and resonates in such a manner that the antenna element and the variable capacitance element cooperate with each other. The power supply terminal supplies the voltage applied to the variable capacitance element. The time constant circuit gradually increases the voltage applied to the variable capacitance element when a voltage applied to the power supply terminal is changed from an off state to an on state.