In the conventional dual-slot Vivaldi antenna, the beam is concentrated in the E-plane by the dual-slot structure, thereby reducing the beam width of the E-plane. Therefore, radiation pattern asymmetry of the E-plane and H-plane occurs. In this work, a double-slot Vivaldi antenna with an extended dielectric structure is proposed to improve the radiation pattern symmetry of the double-slot Vivaldi antenna in the E/H-plane. The proposed antenna reduces the beam width by focusing the E-field on the H-plane in the axial direction of the antenna, using the phenomenon of the refraction of the electric field at the interface between the extended dielectric and air, so that the radiation pattern exhibits symmetry. The effects of reducing the beam width in the H-plane when the extended dielectric is present or absent were compared. It was found that the extended dielectric structure has a beam width reduction effect of as much as 25° according to the frequency.