An antenna device includes: a first variable phase amplifier that outputs a first signal to a first transmission line without outputting a second signal to a second transmission line; a second variable phase amplifier that outputs a fourth signal to a fourth transmission line without outputting a third signal to a third transmission line; a phase comparator that acquires a first reflected signal that is obtained by reflecting the first signal by a first antenna element from the second transmission line, acquires a second reflected signal that is obtained by reflecting the fourth signal by a second antenna element from the third transmission line, and detects a phase difference between the first and the second antenna elements based on the first and the second reflected signals; and a phase amplitude controller that calibrates a phase between the first and the second antenna elements based on the detected phase difference.