A flexible waveguide includes an inner dielectric, and a flexible external conductor disposed in a position covering an outer periphery of the dielectric, the flexible waveguide conducting a radio wave in a frequency band equal to or longer than a millimeter wave or a submillimeter wave near 60 GHz or more. The external conductor includes a metal layer, the metal layer has a shape displacement structure, a shape of an inner periphery side section of which faces the inner dielectric and is cyclic in the waveguide longitudinal direction, the shape displacement structure being a cyclic structure satisfying λmr<；λch, where λmr represents a center wavelength of a main reflection band due to the cyclic structure and λch represents a cutoff wavelength in a high-order mode of the waveguide.
