A method of producing a horn antenna array includes: a step of providing a first die and a second die; a step of assembling the first die and the second die, filling an internal space surrounded by the first die and the second die with a fluid material, and solidifying the fluid material; and a step of, after the material has solidified, separating the first die and the second die. In a portion corresponding to each horn antenna element, the first die has a pair of protrusions and a groove between the pair of protrusions, and the second die has a protrusion. In a state where the first die and the second die are assembled, a gap exists between a tip end of each protrusion of the second die and a bottom face of each groove.