We demonstrate resonantly enhanced transmission of terahertz (THz) wave radiation (gimel similar to 200 mu m) through a bull's eye structure (a single subwavelength aperture surrounded by concentric periodic grooves in a metal plate). The phenomenon is caused by the resonant excitation of surface waves, which are known as surface plasmon polaritons in the optical region, generated by the concentric periodic grooves. Strongly enhanced transmission of THz-wave radiation is observed through the single subwavelength circular aperture (diameterd = 100 mu m). We also show that introducing the Bragg reflector to the bull's eye structure results in the further increase of the enhanced transmission.