We developed a metal hollow waveguide as a flexible delivery medium of terahertz waves. Theoretical evaluation shows that a metal waveguide mainly supports TE modes and that the ${\mathrm{TE}}_{11}$ mode shows a high coupling coefficient to linearly polarized light. Also the ${\mathrm{TE}}_{11}$ mode brings less mode conversion to high-order modes than the ${\mathrm{TE}}_{01}$ mode giving the lowest loss. We measured transmission losses in the terahertz region of hollow waveguides with inner silver coating; the losses were $\sim 7.5–8\mathrm{dB}∕\mathrm{m}$ at the wavelengths from $190–250\mathrm{\mu }\mathrm{m}$ for waveguides with an inner diameter of $1\mathrm{mm}$ . These losses coincide well with theoretical ones, and this shows that in these waveguides, the ${\mathrm{TE}}_{11}$ mode is dominant when a linearly polarized beam is launched into them. The waveguides are flexible because we use a thin-wall glass capillary as the base tubing. Our experiment revealed low additional losses due to bending even when complicated bends were applied to the waveguides. The metal-coated hollow fiber with an inner diameter of $1\mathrm{mm}$ is sufficiently small and flexible for use in endoscopic applications.