An antenna array according to an embodiment includes a conductive member having a first and second slots adjacent to each other. The conductive surface on a front side of the conductive member is shaped so as to define a first and second horns respectively communicating with the first and second slots. The respective E planes of slots are on the same plane, or on a plurality of planes which are substantially parallel to each other. In an E-plane cross section of the first horn, a length from one of two intersections between the E plane and an edge of the first slot to one of two intersections between the E plane and an edge of the aperture plane of the first horn is longer than a length from the other intersection between the E plane and the edge of the first slot to the other intersection between the E plane and the edge of the aperture plane of the first horn, the lengths extending along an inner wall surface of the first horn.