Abstract: A plurality of insulators are disposed in holes, respectively, defined in a metal plate of a waveguide of a circularly-polarized-wave flat antenna. Each insulator has a through-hole and a protrusion. The through-hole extends from the outside of the metal plate to the inside of the waveguide. The protrusion protrudes outward beyond the metal plate. The protrusion has a groove which is open to the outside. Curl antenna elements each have a shaft portion, an arm portion, and a curl portion. The shaft portion is fitted in the through-hole of the insulator and partly protrudes outward from the metal plate. The arm portion extends from the protruded end of the shaft portion. The curl portion is in a substantially helical shape and connected to the top of the arm portion.