An antenna device includes an antenna substrate and a feed line substrate. The antenna substrate includes subarray antennas, feeding interfaces and a back surface. The subarray antennas are arranged parallel with an interval on a plane. Each subarray antenna includes antenna elements and first feed lines. The first feed lines feed signals from the feeding interface on back surface to the antenna elements. The feed line substrate is attached along back surface and includes second feed lines, first and second mode transformers. Each second feed line has one and other ends portions. Other end portion has wider width than one end portion. Each first mode transformer is located in one end portion and connected to the feeding interface. Each second mode transformer is located in other end portion. One end portions are arranged in a line with interval, and other end portions are alternately arranged across one end portions.