We propose and demonstrate a simple leaky structure for terahertz (THz) waveguiding. Different from previously reported air-core THz waveguides, in which a high-reflection-coated cladding layer is designed, the proposed structure here is a simple pipe with a large air core and a thin dielectric layer with uniform but low index. Using commercially available Teflon air pipes, we experimentally confirm that THz waves can be successfully guided in the central air core of 3-m-long pipes with excellent mode qualities, high coupling efficiencies, low attenuation constants, and controllable passband width.