A primary radiator used for a parabolic antenna including: a radiator body having a horn part provided at one end of a waveguide; and a waterproof cover covering an open end of the horn part of the radiator body, wherein a step is formed on an inner surface of the horn part of the radiator body, and a position of the step is set so that radio waves reflected on the waterproof cover are cancelled out by radio waves reflected on the step to prevent multiple reflection in the radiator body.