Transformation and inverse transformation between a free-space linearly polarized beam and the vectorial vortex mode of a circular hollow waveguide by use of Pancharatnam–Berry phase optical elements is proposed. Demonstration was achieved by fabricating $\mathrm{GaAs}$ subwavelength gratings and utilizing a $300\mathrm{\mu }\mathrm{m}$ diameter hollow metallic waveguide for $10.6\mathrm{\mu }\mathrm{m}$ wavelength ${\mathrm{CO}}_2$ laser radiation. The mode transformations and the excitation of a single vectorial mode inside the hollow waveguide were verified by full polarization measurements. In addition, the inverse mode transformation of the single vectorial mode excitation in the waveguide enabled us to experimentally obtain a linearly polarized bright spot with a high central lobe.