The feed-horn with helical antenna element of this invention comprises (1) a waveguide made of conductive cylinder having a step inside, the inner diameter in the vicinity of the opening face of the waveguide is larger than that of the base of the waveguide, and (2) a helical antenna element disposed at the base axial center of the waveguide. Introduction of the step-formed inside to the waveguide makes it possible to obtain excellent characteristics both of cross polarization and impedance.